Library with LOTS of clips: Increasing performance
Ideally having all your content and project files on a separate harddisk would give you some performance back, this could be a internal drive if your on a Apple Mac Pro or a external drive, USB3, Fire-wire, Thunderbolt etc.

If you dont need to have the libraries open I would close them as they load up the files when you open FCPX.

Other options are to increase your RAM in the Mac, FCPX likes a min of 4gb, I had 6gb in mine and it was running slow, I upped it to 16gb and now it runs so much better, I also made sure I was running the latest FCPX version and I closed some of my libraries down which I didnt need to have open all the time as you can just re-open them when you need to.

I mainly do weddings where I shoot a lot of clips throughout the day 200+ sometimes so it shouldn't have an issue really. I do keep my projects in a very strict order and filing system
Library Name
--Event Name (normally one for RAW files, Audio, Titles etc etc)
--Project Names (normally one for each timeline)

I can have 3/4 edits going at any one time, once I finish one I close it down and archive the whole library removing any render files, optimized media etc to keep the archive size down.

I hope something in this will help