I’ve now edited several complex feature-length docs w/FCP back to back with high shooting ratios. FCP doesn’t always go smoothly with large projects, it’s true.
I’ve got into the habit of splitting them down into separate projects which you can have open simultaneously, thereby keeping the individual project file size down. Break them down initially into Media/Edits, then further in any way that seems logical (Slates, Interviews etc etc). It’s no problem copying and pasting from one project into another as you wish. It’s a workflow which I’ve managed to adopt with minimum hassle.
This won’t solve your speed changes (unfortunately), nor is this a solution for those working in multi-editor setups, but it’s certainly worth a try for any project that’s getting too unwieldy.