Go to Preferences and do next:
Appearance & Behaviour > System Settings > Updates: disable auto update
Appearance & Behaviour > System Settings > Using Statistics: Uncheck allowing sending data
Editor > Live Templates: disable all, leave only what you are really use
Editor > Emmet: disable all emmets
Editor > Intentions: I leave only: CSS, Declaration, JavaScript and Language Injection
Plugins: leave only next (* - can be also disabled in case you don't need them):
CoffeeScript *
CSS Suport
CVS Integration
Git Integration
HTML Tool
IntelliLang
JavaScript Debugger *
JavaScript Intention Power Pack
JavaScript Support
NodeJS *
Perforce Integration
SASS suport *
Project > Directories: Exclude all what you don't use
Languages & Frameworks > JavaScript > Libraries: leave only: HTML and HTML5 / EcmaScript 5
Languages & Frameworks > Compass: disable it
Tools > WebBrowsers: leave only Chrome
Open terminal, mc or what you prefer and edit vmoptions and increase usage memory pwd: "/Applications/WebStorm.app/Contents/bin/webstorm.vmoptions"

-Xms1024m
-Xmx1536m
-XX:MaxPermSize=1024m
-XX:ReservedCodeCacheSize=512m
-XX:+UseCompressedOops