This is actually a bug for a long time which still is occurring regularly.
If the browser is open, things run smoothly but if the computer goes in sleep mode, at some (unpredictable) moment LEDs start updating very slowly, sometimes only a few frames per minute.

theories

• esp32 goes into sleep mode
• LEDs loop is in the main loopTask which has too low priority
• Data from the server to the UI is queueing up

This has never happened in StarLight (also loop in the main queue)