Edit 2017: You may first want to try experimenting with allocating PhpStorm some more RAM if available. Do this by

Going to Help -> Edit Custom VM Options Change -Xms and -Xmx to be something reasonable for your hardware. I have -Xms512m and -Xmx2048m on an 8GB RAM laptop. Restart PhpStorm. If you want to see how much RAM PhpStorm is currently using, you can go to File -> Settings and search for and enable show memory indicator .

Original answer: I made some changes to increase the responsiveness of the IDE. I'm sure there are more things one could do, but I found these to improve the performance to well within usable margins. From most effective to least:

Disable language injections: File -> Settings -> Language injections. Untick as many boxes as you're comfortable with. HTML was the real killer for me. Disable inspections: File -> Settings -> Inspections. Untick as many as you don't need. Disable unused plugins: File -> Settings -> Plugins. Untick unused.

These changes brought down both the startup time and significantly increased responsiveness of the IDE in general.