slow speed

That's not cool! I'd love to help you get to the bottom of things. 
First things first...

• Can you please share your system specs, GPU, and OS version? 
• Can you reproduce this issue in the latest release version of Premiere Pro 25.1
• Can you please share a log so devs can take a peek for any suspicious clues? 
  • Start an app session 
  • Get into the bad playback state 
  • Navigate to Help > Reveal Log Files and send those files over. 
• Screen Recordings also help a great deal to be able to see what panels you have open, etc

Out of curiosity, have you tried the new Media Intelligence feature? 

• Can you please go into Prefs > Media Intelligence & Transcripts and turn Visual Analysis off by unchecking the box? 
• 

• Does this improve playback?  

Thanks for reporting this issue and for any further detail you can provide,