Thanks Bob for all of your good suggestions.

I've tried playing around with some of the TCP/IP tuning. I also
played around with the max-connections in Explorer and Firefox. Not
much difference in performance. I did swap a unshielded serial cable
(the radios are directly connected to the computers' serial ports) with
a shielded cable, and that seemed to improve the performance. The
serial lines are fairly long (60') to get to the antennas which are
colocated with the radios. I would say the system is usable (slow)
with a browser, but not good with heavy traffic, which results in
significantly more retransmissions and reset connections.

The reason why the link is disappointingly slow is that it only runs at
19 kbps. According to the Aerocomm AC4490 manual, the maximum
throughput of the radio in stream mode is 57.6 kbps, acknowledge mode
is 38k and acknowledge with full/duplex is 19k (full/duplex is only
supported in acknowledge mode). Running PPP or SLIP requires the
full/duplex hardware, so 19k is all I can get.

I'm in the market for better but inexpensive 900 Mhz radios to replace
the Aerocomms, if anyone has any suggestions.