That’s not very scientific though.
It might have been just one plugin causing the issue.
However, agree with Stephen that 21 is a lot!
You need to use discretion when installing a plugin. Ask yourself important questions like:
- How critical is the functionality to my community?
- Is the plugin well supported? (judge by the plugin topic history)
- Is the plugin of high quality, is it stable? (Ditto)
Keep them to a minimum.
Having half a dozen high quality plugins is reasonable.
But just because you can eat 21 doughnuts 
doesn’t mean you should …