In my opinion, OS X Yosemite is not quite as stable as previous versions of OS X (Mavericks, Mountain Lion, etc). Part of that is because Apple seems to be spread thin between OS X, iPhone, iPad, Apple Watch, Apple TV... they have a lot going on right now. Unfortunately, that seems to have a negative impact on the overall quality of OS X.
One of the areas where Yosemite seems to be falling short is how it handles memory management. Iv'e seen a number of cases where Mac users complain of Mail, Calendar and other apps taking up a massive amount of processing power after its been open for just a few hours. The short-term fix for this is to Quit the affected app (you can find out which app is having trouble by looking at Activity Monitor) and re-open. The downside to this method is that you'll be right back in the same spot within a few hours (usually). Here's a fix that seems to remedy that problem... permanently.
I created this set of instructions so that really any user (from beginner to expert-level) can follow them. However, if you feel uncomfortable with doing some of these steps, let me know and I can hold your hand. Here we go!
- Disconnect all externally-connected devices (including hard drives, scanners, screens, etc).
- Turn off your Mac.
- Once your Mac is off, enter Safe Mode by turning on your Mac and pressing the Shift key when you hear the startup chime (release when you see the grey status bar).
- While in Safe Mode, open Disk Utility (Finder > Applications > Utilities), highlight your Macintosh HD partition on the left, and click “Repair Disk Permissions".
- After Repair Disk Permissions have finished, turn off your computer.
- Reset your System Management Controller (SMC) by following these instructions.
- Once your Mac has completed a full boot after resetting the SMC, turn it off again.
- Reset your PRAM by following these instructions.