Go into Preferences > Settings. Add:

"index_workers": 2

References:

• https://forum.sublimetext.com/t/sublime-text-3-is-very-slow/19596/6
• https://forum.sublimetext.com/t/solved-high-cpu-usage-in-beta-3124/23060