Q: OS X Yosemite v10.10.5 Update

Try running the combo update.

10.10.5 Combo Update

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

6 Easy Tips to Speed Up OS X Yosemite on Your Mac

Try running this program and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read.

Etrecheck – System Information