A large amount of Tokens slows down everything - Placeholder Tokens
You can find some solutions for such a problem on Node references + Entity Tokens creates HUGE Token Tree UI table HTML.

Using the Token tweaks module also helps. I've tried and it works fine.


  Provides tweaks for tokens (mostly to improve performance)
  
  
  Disable token types or specific tokens.
  Modify the default depth limit for the token tree UI.