They're all pretty CPU heavy. My computer is old and struggles with amp sims. Here's what I do when I use them:
Mute everything that doesn't need to be running to track guitars. Record a direct guitar track with the sim and save it. This is the basic raw guitar track. Duplicate that basic guitar track, mute the original guitar track, dial in the tone you want on the duplicated track and burn the sim on to the track so it's permanent. Turn the sim on the duplicated track off, and you're left with a track that's like you recorded it from an amp with no CPU power being eaten up. If you need to tweak the tone, go back to your original guitar track, make another duplicate and do it all over again. Do this for each of your guitar tracks. Just make sure you always have the raw, direct guitar track saved and muted so you can go back and tweak as much as necessary while never having to leave sims running and eating up CPU power. It seems like a retarded amount of steps and work, and it is, but it works for older, less powerful machines.