Hi Martin--Amplitube 3 has three different quality modes, which can be set from the main window on the upper right. The manual describes them as follows:
"HI:offers the best sound quality at a higher CPU usage
MID:offers a very good sound quality at a reasonable CPU usage
ECO:offersa good sound quality at a very low CPU usage"
Here's another suggestion. If your DAW allows for "comping" tracks, that is, recording multiple takes on the same track and the putting together the best track using sections from any of the takes, then you don't have to record each one on a separate track with a separate instance of Amplitube. If you have, say, three different guitar parts in a song (maybe two rhythms and a lead), you'd only need three guitar tracks total, each with one instance of AmpliTube.
If that doesn't solve your problem, you can bounce individual guitar tracks to disk, with AmpliTube inserted on them, and then put together your parts from the bounced tracks, all of which have AmpliTube's effects on them already, so they no longer require the plug-in to be inserted. Some DAWs also have a "Freeze Tracks" function, which, when you activate it for a track, temporarily renders the track with whatever plug-ins are on it, and turns off the plug-ins to save CPU.