Is an admin password enough to protect my files? So I'm debating whether or not I should encrypt my MacBook Pro's HDD. Is the account password enough to keep my files safe? If my MacBook got stolen would the thief be able to access the files?

Would encrypting slow things down significantly?
In general, no - setting an admin password is not enough to keep your files safe. If someone has possession of your MBP, it is pretty easy for them to change your account password, and then access any of your unencrypted files. Your keychain is safe, since it cannot be unlocked without the original password.

FileVault 2 (Full Disk Encryption) does not make much of an impact on speed. If security is a concern, it is worth implementing.