diff --git a/copyparty/__main__.py b/copyparty/__main__.py
index d83b5b2c..1998eff7 100755
--- a/copyparty/__main__.py
+++ b/copyparty/__main__.py
@@ -869,6 +869,7 @@ def add_fs(ap):
     ap2 = ap.add_argument_group("filesystem options")
     rm_re_def = "5/0.1" if ANYWIN else "0/0"
     ap2.add_argument("--rm-retry", metavar="T/R", type=u, default=rm_re_def, help="if a file cannot be deleted because it is busy, continue trying for \033[33mT\033[0m seconds, retry every \033[33mR\033[0m seconds; disable with 0/0 (volflag=rm_retry)")
+    ap2.add_argument("--iobuf", metavar="BYTES", type=int, default=256*1024, help="file I/O buffer-size; if your volumes are on a network drive, try increasing to \033[32m524288\033[0m or even \033[32m4194304\033[0m (and let me know if that improves your performance)")
 
 
 def add_upload(ap):
diff --git a/copyparty/ftpd.py b/copyparty/ftpd.py
index 4e349401..a918b234 100644
--- a/copyparty/ftpd.py
+++ b/copyparty/ftpd.py
@@ -218,7 +218,7 @@ class FtpFs(AbstractedFS):
                 raise FSE("Cannot open existing file for writing")
 
         self.validpath(ap)
-        return open(fsenc(ap), mode)
+        return open(fsenc(ap), mode, self.args.iobuf)
 
     def chdir(self, path: str) -> None:
         nwd = join(self.cwd, path)
diff --git a/copyparty/httpcli.py b/copyparty/httpcli.py
index 3bd98e8c..b13f2dd4 100644
--- a/copyparty/httpcli.py
+++ b/copyparty/httpcli.py
@@ -174,7 +174,6 @@ class HttpCli(object):
         self.parser: Optional[MultipartParser] = None
         # end placeholders
 
-        self.bufsz = 1024 * 32
         self.html_head = ""
 
     def log(self, msg: str, c: Union[int, str] = 0) -> None:
@@ -1641,7 +1640,7 @@ class HttpCli(object):
         bos.makedirs(fdir)
 
         open_ka: dict[str, Any] = {"fun": open}
-        open_a = ["wb", 512 * 1024]
+        open_a = ["wb", self.args.iobuf]
 
         # user-request || config-force
         if ("gz" in vfs.flags or "xz" in vfs.flags) and (
@@ -1900,7 +1899,7 @@ class HttpCli(object):
         f.seek(ofs)
         with open(fp, "wb") as fo:
             while nrem:
-                buf = f.read(min(nrem, 512 * 1024))
+                buf = f.read(min(nrem, self.args.iobuf))
                 if not buf:
                     break
 
@@ -2162,7 +2161,7 @@ class HttpCli(object):
                     except:
                         pass
 
-            f = f or open(fsenc(path), "rb+", 512 * 1024)
+            f = f or open(fsenc(path), "rb+", self.args.iobuf)
 
             try:
                 f.seek(cstart[0])
@@ -2185,7 +2184,8 @@ class HttpCli(object):
                     )
                     ofs = 0
                     while ofs < chunksize:
-                        bufsz = min(chunksize - ofs, 4 * 1024 * 1024)
+                        bufsz = max(4 * 1024 * 1024, self.args.iobuf)
+                        bufsz = min(chunksize - ofs, bufsz)
                         f.seek(cstart[0] + ofs)
                         buf = f.read(bufsz)
                         for wofs in cstart[1:]:
@@ -2482,7 +2482,7 @@ class HttpCli(object):
                         v2 = lim.dfv - lim.dfl
                         max_sz = min(v1, v2) if v1 and v2 else v1 or v2
 
-                    with ren_open(tnam, "wb", 512 * 1024, **open_args) as zfw:
+                    with ren_open(tnam, "wb", self.args.iobuf, **open_args) as zfw:
                         f, tnam = zfw["orz"]
                         tabspath = os.path.join(fdir, tnam)
                         self.log("writing to {}".format(tabspath))
@@ -2778,7 +2778,7 @@ class HttpCli(object):
         if bos.path.exists(fp):
             wunlink(self.log, fp, vfs.flags)
 
-        with open(fsenc(fp), "wb", 512 * 1024) as f:
+        with open(fsenc(fp), "wb", self.args.iobuf) as f:
             sz, sha512, _ = hashcopy(p_data, f, self.args.s_wr_slp)
 
         if lim:
@@ -3010,8 +3010,7 @@ class HttpCli(object):
             upper = gzip_orig_sz(fs_path)
         else:
             open_func = open
-            # 512 kB is optimal for huge files, use 64k
-            open_args = [fsenc(fs_path), "rb", 64 * 1024]
+            open_args = [fsenc(fs_path), "rb", self.args.iobuf]
             use_sendfile = (
                 # fmt: off
                 not self.tls
@@ -3146,6 +3145,7 @@ class HttpCli(object):
 
         bgen = packer(
             self.log,
+            self.args,
             fgen,
             utf8="utf" in uarg,
             pre_crc="crc" in uarg,
@@ -3223,7 +3223,7 @@ class HttpCli(object):
         sz_md = 0
         lead = b""
         fullfile = b""
-        for buf in yieldfile(fs_path):
+        for buf in yieldfile(fs_path, self.args.iobuf):
             if sz_md < max_sz:
                 fullfile += buf
             else:
@@ -3296,7 +3296,7 @@ class HttpCli(object):
             if fullfile:
                 self.s.sendall(fullfile)
             else:
-                for buf in yieldfile(fs_path):
+                for buf in yieldfile(fs_path, self.args.iobuf):
                     self.s.sendall(html_bescape(buf))
 
             self.s.sendall(html[1])
diff --git a/copyparty/star.py b/copyparty/star.py
index 5a4663ba..6d93f442 100644
--- a/copyparty/star.py
+++ b/copyparty/star.py
@@ -1,6 +1,7 @@
 # coding: utf-8
 from __future__ import print_function, unicode_literals
 
+import argparse
 import re
 import stat
 import tarfile
@@ -44,11 +45,12 @@ class StreamTar(StreamArc):
     def __init__(
         self,
         log: "NamedLogger",
+        args: argparse.Namespace,
         fgen: Generator[dict[str, Any], None, None],
         cmp: str = "",
         **kwargs: Any
     ):
-        super(StreamTar, self).__init__(log, fgen)
+        super(StreamTar, self).__init__(log, args, fgen)
 
         self.ci = 0
         self.co = 0
@@ -126,7 +128,7 @@ class StreamTar(StreamArc):
         inf.gid = 0
 
         self.ci += inf.size
-        with open(fsenc(src), "rb", 512 * 1024) as fo:
+        with open(fsenc(src), "rb", self.args.iobuf) as fo:
             self.tar.addfile(inf, fo)
 
     def _gen(self) -> None:
diff --git a/copyparty/sutil.py b/copyparty/sutil.py
index 784e122c..41a5270d 100644
--- a/copyparty/sutil.py
+++ b/copyparty/sutil.py
@@ -1,6 +1,7 @@
 # coding: utf-8
 from __future__ import print_function, unicode_literals
 
+import argparse
 import os
 import tempfile
 from datetime import datetime
@@ -20,10 +21,12 @@ class StreamArc(object):
     def __init__(
         self,
         log: "NamedLogger",
+        args: argparse.Namespace,
         fgen: Generator[dict[str, Any], None, None],
         **kwargs: Any
     ):
         self.log = log
+        self.args = args
         self.fgen = fgen
         self.stopped = False
 
diff --git a/copyparty/szip.py b/copyparty/szip.py
index 85d5a9cc..de6b0851 100644
--- a/copyparty/szip.py
+++ b/copyparty/szip.py
@@ -1,6 +1,7 @@
 # coding: utf-8
 from __future__ import print_function, unicode_literals
 
+import argparse
 import calendar
 import stat
 import time
@@ -218,12 +219,13 @@ class StreamZip(StreamArc):
     def __init__(
         self,
         log: "NamedLogger",
+        args: argparse.Namespace,
         fgen: Generator[dict[str, Any], None, None],
         utf8: bool = False,
         pre_crc: bool = False,
         **kwargs: Any
     ) -> None:
-        super(StreamZip, self).__init__(log, fgen)
+        super(StreamZip, self).__init__(log, args, fgen)
 
         self.utf8 = utf8
         self.pre_crc = pre_crc
@@ -248,7 +250,7 @@ class StreamZip(StreamArc):
 
         crc = 0
         if self.pre_crc:
-            for buf in yieldfile(src):
+            for buf in yieldfile(src, self.args.iobuf):
                 crc = zlib.crc32(buf, crc)
 
             crc &= 0xFFFFFFFF
@@ -257,7 +259,7 @@ class StreamZip(StreamArc):
         buf = gen_hdr(None, name, sz, ts, self.utf8, crc, self.pre_crc)
         yield self._ct(buf)
 
-        for buf in yieldfile(src):
+        for buf in yieldfile(src, self.args.iobuf):
             if not self.pre_crc:
                 crc = zlib.crc32(buf, crc)
 
diff --git a/copyparty/tftpd.py b/copyparty/tftpd.py
index 86d06538..6c650951 100644
--- a/copyparty/tftpd.py
+++ b/copyparty/tftpd.py
@@ -340,6 +340,9 @@ class Tftpd(object):
         if not self.args.tftp_nols and bos.path.isdir(ap):
             return self._ls(vpath, "", 0, True)
 
+        if not a:
+            a = [self.args.iobuf]
+
         return open(ap, mode, *a, **ka)
 
     def _mkdir(self, vpath: str, *a) -> None:
diff --git a/copyparty/up2k.py b/copyparty/up2k.py
index dcf364cc..39f386a0 100644
--- a/copyparty/up2k.py
+++ b/copyparty/up2k.py
@@ -3920,7 +3920,7 @@ class Up2k(object):
         csz = up2k_chunksize(fsz)
         ret = []
         suffix = " MB, {}".format(path)
-        with open(fsenc(path), "rb", 512 * 1024) as f:
+        with open(fsenc(path), "rb", self.args.iobuf) as f:
             if self.mth and fsz >= 1024 * 512:
                 tlt = self.mth.hash(f, fsz, csz, self.pp, prefix, suffix)
                 ret = [x[0] for x in tlt]
diff --git a/copyparty/util.py b/copyparty/util.py
index f9401790..18cf14a2 100644
--- a/copyparty/util.py
+++ b/copyparty/util.py
@@ -2361,10 +2361,11 @@ def build_netmap(csv: str):
     return NetMap(ips, cidrs, True)
 
 
-def yieldfile(fn: str) -> Generator[bytes, None, None]:
-    with open(fsenc(fn), "rb", 512 * 1024) as f:
+def yieldfile(fn: str, bufsz: int) -> Generator[bytes, None, None]:
+    readsz = min(bufsz, 128 * 1024)
+    with open(fsenc(fn), "rb", bufsz) as f:
         while True:
-            buf = f.read(128 * 1024)
+            buf = f.read(readsz)
             if not buf:
                 break
 
diff --git a/scripts/sfx.py b/scripts/sfx.py
index 949f85d1..7af2f905 100644
--- a/scripts/sfx.py
+++ b/scripts/sfx.py
@@ -234,8 +234,9 @@ def u8(gen):
 
 
 def yieldfile(fn):
-    with open(fn, "rb") as f:
-        for block in iter(lambda: f.read(64 * 1024), b""):
+    s = 64 * 1024
+    with open(fn, "rb", s * 4) as f:
+        for block in iter(lambda: f.read(s), b""):
             yield block
 
 
diff --git a/tests/util.py b/tests/util.py
index 505452d4..8cfa61ea 100644
--- a/tests/util.py
+++ b/tests/util.py
@@ -147,6 +147,7 @@ class Cfg(Namespace):
             dbd="wal",
             fk_salt="a" * 16,
             idp_gsep=re.compile("[|:;+,]"),
+            iobuf=256 * 1024,
             lang="eng",
             log_badpwd=1,
             logout=573,