If you notice that you’re running out of RAM on OSX Yosimite…
First, open you Activity Monitor, click the [Memory] tab and sort by “Memory”
If WindowServer is hogging your RAM then resetting you mac’s PRAM might fix things.

Reset your mac’s PRAM:
-Close all open apps
-Press CMD + Option + ESC (***)
-Put your mac to Sleep
-Wake you mac
-Restart your mac
-Press CMD + Option + P + R (keep it down until you hear at least 2 chimes)

(***) Here’s where you force quit anything left open. It’s okay to leave Finder open

Notes:
If you also have excessive CPU utilization, you may want to Repair Disk Permissions, which adds a step to the above process

Reset Disk Permissions and your mac’s PRAM:
-Close all open apps
-Press CMD + Option + ESC (***)
-Put your mac to Sleep
-Wake you mac
-Restart your mac
-click “Repair disk permissions”
-Restart your mac
-Press CMD + Option + P + R (keep it down until you hear at least 2 chimes)