I have done some tests and I do not see any speed difference even with 1MB buffer.
I can send you a custom build if your want to test in your environment. Contact me at my email address.

Note that the OS-level buffer is already set to 4MB. That's probably why the application buffer size has little if any impact.

Thanks for your suggestion. I'll look into it.

the current value is fixed value 16KB in
source/filezilla/TransferSocket.cpp

It's too small for large file transfer in fast LAN/WAN network.
It would be good we could config the value up to 4MB.