Record to same track

[steve]

Muting channels does very little to reduce the load.
The "mixing engine" in Audacity is extremely efficient. The thing that takes a lot of processing power is drawing (and redrawing) the waveform.

The way to reduce the computer load is to reduce the number of tracks, which can be done by "mixing down" some (or all) of the tracks to a lesser number of tracks. The disadvantage in doing this is that once mixed down, tracks cannot be "un-mixed" (other than using "Undo" while the project remains open).

I think that we are all struggling to answer your question because we are not sure what your question is. Perhaps if you could try to describe the job in detail we may be able to provide more pertinent answers.