I recently turned FileVault off because I really saw no need for it on my home computer (Mac Intel i5 16GB RAM w/Maverick). However, I noticed when I tried running Mac Booster 3 (a cleanup/performance app), it took forever (9+ hrs) to run and I'm wondering if it's because the files are still encrypted? Does turning FileVault off leave the files encrypted or not?
No, your disk is decrypted when you turn off FileVault.
From the Apple Knowledgebase:
When you turn off FileVault, encryption is turned off and the contents of your disk are decrypted. The decrypting of your disk could take a while, depending on how much information you have stored. However, you can still use your Mac to do other tasks while the decryption is occurring.
It could be that FileVault was still decrypting your hard drive while you were running Mac Booster.