My machine had been slow for quite a while and upon investigating the cause it became obvious that something was causing Sublime to be stuck in an indexing loop (using spikes of 100% CPU, then going idle for 30 seconds or so).

The culprit was:

reloading python 3.3 plugin Naomi.__init__
generating meta info summary
error parsing lexer: Packages/Naomi/syntaxes/css3/combinator.sublime-syntax: invalid type for match at line 24 column 14

Disabling Naomi resolves the issue.

The issue has been present in Sublime Text 4 builds 4124, 4123, and 4122 at least.

NB: It seems the Naomi project might be dead? But this information may help someone Googling for a solution.