PhpStorm can be very slow in large files as its performing so many inspections. One quick and easy way to speed up PhpStorm is to render using OpenGL. Previously in a 5000 line file it would give the 'eye' symbol in the top right for a long time before changing to a tick (or red/yellow box). After OpenGL it does this almost immediately.

To enable OpenGL:

Open: path-to-phpstorm\bin\PhpStorm64.exe.vmoptions

Then add these two lines below the others:

-Dawt.useSystemAAFontSettings=lcd 
-Dawt.java2d.opengl=true

Its possible to change *.vmoptions and idea.properties files without editing them in the PhpStorm installation folder.

Follow the steps below:

Step 1:

Run Help - Edit Custom VM Options...

Step 2:

Confirm the creation of the configuration file, if prompted

Step 3:

Add following lines if you want to use OpenGL

# This line could already be there depending on your PHPStorm version
-Dawt.useSystemAAFontSettings=lcd 

-Dawt.java2d.opengl=true

Add the following lines if you want to increase memory allocated to PhpStorm (improves performance in large projects)

# This line could already be there depending on your PHPStorm version
-Xmx750m

Save the file and restart program.

If you're in a hurry and want to quickly get a speed boost PLUS save some power, go into Power Save Mode. It's in the File menu, bottom-most option in that menu.

File -> Power Save Mode

Note: this will disable some powerful features like syntax highlighting, code analysis, auto-completing things. It will also stop indexing.