As it turns out, there is a way to fix the combat lag. Quite simple, actually.
- Run the Fallout 2 Resolution configuration program (F2_res_config.exe or something similar in the installation directory)
- Click "Manually edit config"
- Find the line that says "EXTRA_WIN_MSG_CHECKS=0". Change the 0 to a 1 and save it.
Thanks to this thread on the Steam forums for this info!