I'm going to guess that it might take several minutes to load, even with the toolpath display off. Just wait.
I believe Mach3 can load 10 million lines of code.
On my 1Ghz PIII, I have to turn off the toolpath on anything over 250,000 lines, or the PC is very slow to respond, and movements can be erratic. With the toolpath off, everything runs smoothly.