Why does Yosemite keep freezing?

Question

I have a MacBook Pro, 2012 I believe. For the most part, it's been one of the best computers I've owned so far, running smoothly with few glitches or errors. Then I updated to Yosemite.
Now, I'm lucky if a job that took a few seconds on the previous system takes less than five minutes on the new one. The system lags and hangs pretty much every other action. It has to 'think' about everything, but especially typing. While writing this, the system paused for over fifteen seconds, unable to do anything, before continuing with typing out my sentence. Here's a little bit of what keeps going wrong:
1.) While typing in Microsoft Word, the system will continually and consistently freeze, often in the middle of a word I was tying, sometimes for upwards of a minute. May or may not complete the word or phrase I was typing when freeze occurred (literally just happened while typing this sentence; so Firefox is buggy too) Saving and opening files also takes an inordinate amount of time.
2.) Using iTunes or 3rd Party software for music and/or videos is pretty much impossible. Audio will repeatedly pause and glitch for upwards of thirty seconds before resuming, or just cut out entirely. Video doesn't fare much better, however video will sometimes freeze while audio continues on for a few seconds before freezing as well. Other times video and audio will pause at once and then resume. Problem is also present (but less pronounced) for YouTube videos.
3.) Accessing files takes upwards of a minute (depending on size and location) and will necessitate no other activity on other programs in the meantime (system will lock up temporarily and then resume as file opens).
4.) System will often stop and lock up even when I'm not doing anything; evidenced by the spinning 'busy' icon appearing and the system not responding when prompted.
5.) System seems to have a problem with scrolling through menus or websites as well.
6.) Possibly the cause of -or related to- these problems: Filevault will not stop its encryption process. I left the system running on cord power (modifying the appropriate energy-saving settings so the computer wouldn't go to sleep automatically) for upwards of 24 hours with no programs running, and the progress bar hasn't moved. (Occasionally it'll jump to full and then return to half-full)
I love my computer, but this new OS has made me pull my hair out and want to bash my head into the wall, repeatedly. I have no idea how to fix this, or how I would implement a solution, if any. My only thought is that I should re-download and install the previous OS, but it'd be a tremendous hassle and I'm worried about compatibility issues with some of the newer files. Could someone please explain why this keeps happening and walk me through a solution? Bear in mind that I am the DEFINITION of 'Stupid end-user'. I know nothing about how computers or coding works, and I'm just trying to get mine fixed.

Answer 1

Activity Monitor - Mavericks  also Yosemite
Activity Monitor in Mavericks has significant changes
Performance Guide
Why is my computer slow
Why your Mac runs slower than it should
Slow Mac After Mavericks
Things you can do to resolve slowdowns  see post by Kappy
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