Maybe I am thinking this wrong...
If it was up to me... What if you choose your real tracks, assuming to the bin (mixer). Program renders them to a lower quality "temp" files that is enough to work out the idea/sketch. For example, original files 100megs Temp low size files would be something like 10.

Having to render a lower quality files will make rendering process to much (MUCH!!) faster with option to render full weighted song. Meaning all the changes you make are just "changes" that will be saved to be used with small size temp files or with original files. This was the approach Adobe took in their video editing software Looooooooooong time ago.

And of course it should be possible to render just "selection" of timeline when working with full size files.

just my 2 cents smile