Thank you all for the helpful suggestions. Here's more information:
Turning off the toolpath display made little or no difference. (Too bad -- I was hoping that would solve it).
The Task Manager shows the activity in both CPUs. While Mach3 is running, the graphs show small spikes in both CPUs down in the single digits. System Idle is 99% on the average (no other background processes are eating CPU time). Mach3.exe is the only additional process that shows up when Mach3 is launched, and it shows 7 threads running. Total memory used by Mach3.exe is only about 63M. There is plenty of available memory.
I think this shows that it's not a CPU problem or the need for an external graphics card.
What is it in Mach3 that schedules and calls the LookAhead calculations? It's not calling for the lookahead calculations fast enough even though there is plenty of CPU time available. Even on a simple program, when I start running it you can watch the queue depth counter counting up by about 5 per second. It doesn't matter how large I make the LookAhead, the queue depth never gets too high on any program I run. It doesn't matter if the g-code uses g93 or g94.
Any more suggestions?