PyCharm is terribly slow when editing HTML / CSS / JS
There are two solutions:

You can disable HTML / CSS / JS inspections in Preferences -> Editor -> Inspections.
If your project has a decent amount of third-party libraries (jquery, bootstrap etc.), this might be the reason. Don't index directories with these libraries: right click on a folder and then choose Mark Directory As -> Excluded.

Don't forget to clear the cache: File -> Invalidate Caches / Restart.
PyCharm becomes nearly unresponsive in 5 minutes of editing HTML / CSS / JS code – even typing a single character causes a several seconds lag. 
However, everything is absolutely fine when I work with Python. I don't close PyCharm for days and it works smooth.
Restarting PyCharm helps only for a few minutes. I'm on PyCharm 5.0.5 Professional & OS X 10.11.