On the hard disk itself, in Reaper's Project Templates folder, my largest "master" template currently has 358 tracks, and the .rpp weighs in at 163 MB. It loads virtually instantaneously in its disabled state (about the same as empty tracks would), and in Reaper's Performance tab in the dock it shows CPU hovering between approx. 0.6% and 1.0% (i7 2600).
The utilized RAM displays as 454 MB. Just as an experiment, I enabled 6 selected Kontakt 5 Spitfire Albion instrument tracks (with a single click) and the memory usage then jumped to about 3 GB.
Disabling and locking a track in Reaper is different than freezing it, though I don't know the nuances of how that all works in terms of resource consumption. All I know is that I can have fairly large templates without long waiting times and without fuss, and that performance and resource limitations do not seem to come into play until the active project itself gets too large, IOW too full of fully-unlocked, enabled tracks.
EDIT: I haven't analyzed saving times, but it hasn't been a noticeable issue, as I believe that when saving the time is only affected by the enabled tracks active in the project, and that the disabled, locked and unused tracks do not add to the amount of time it takes to save a project. Or if they do, it's not significant.