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.
Without encryption, yes, a malicious actor with physical access to your machine can easily access your files. Permissions only stop users who are abiding by the rules imposed by the OS.
Full disk encryption is fast enough to be basically transparent. It does of course have some cost, but if you want to make it difficult for someone with access to your machine to get at your data full disk encryption is your simplest answer.
An alternative to full disk encryption is make a new encrypted disk image file with Disk Utility, and keep your sensitive files in it. While the rest of your files would be still be vulnerable to exposure to someone in possession of your computer or its hard disk, most of us can identify a relatively small subset of files or folders of files that need more protection. Email, financial data, perhaps your calendar, come to mind; most of the rest of the contents of your ~/Library, the system files, applications your music, virtual hard disks and other non-personal files mostly aren't worth encrypting. You might not need to open the 'vault' every session if access to those files isn't your primary mission.
Of course if you manage/store/transport other people's private information on your machine, it is probably what occupies most of your disk space, and you need the best software and physical security available.