25% gain here Rated 3 out of 5 stars

I've tested 'Firefox Preloader 1.0' on Firefox 6.02 and now on Firefox 7 Beta6, with 60+ add-ons. Platform is Windows XP SP3, computer is old (Duron CPU at 1.6 gHz!), Windows Prefetch is on for boot only (hence Firefox not concerned):
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Cold starts of course,
FF 6.02: Without Preloader: 21sec -- With Preloader: 16sec
FF 7 Beta6: Without Preloader: 12sec -- With Preloader: 9sec
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That leads to a gain of 25% with Preloader;
Shows as well how faster to start Firefox 7 is. Extra delay on boot with Preloader is not noticeable (does exist slightly perhaps but perception of time is as we know subjective)

This review is for a previous version of the add-on (1.0).