In pure DOS, using mTCP and spdtest, DE22XPD and 3C90XPD drivers.
Having the P233 be the sender and the P120 be the listener, it reaches 8 Mbps (1000 KB/s),
Switching places, P120 sender, P233 listener, I get 5.3 Mbps (662.5 KB/s).
P120 mTCP FTP download yields 6.82 Mbps (853KB/s), I guess there's losses due to overhead for FTP or it could be my FTP server expects faster clients as pointed out before.
But for this machine, I think that's as close as I'm going to get.
Update:
With the Cyrix 486-66 using mTCP FTP and the Realtek 8019 Packet Driver (which looks identical to the Crynwr driver btw) I reached 6.4 Mbps (800 KB/s)
I think this is as fast as they're both going to get, though I am still surprised that the P120 still couldn't max out the card, but maybe it's not the CPUs fault. Who knows?
Thanks to everyone who replied, I'm still interested to see what everyone else "benchmarks" on low end machines and 10 Mbit ISA cards though.
Maybe we can build a matrix of card, driver and CPU with speeds to record our findings?
Twisted.
My garage, 15 PCs from 1990 to 2020, 486 to 5900X - https://www.thecodecache.net