I upgraded to Tutor 19.0.5 after seeing this forum post Performance Issue in Studio with Large Courses – Multiple Clipboard API Calls Causing Browser Crash - #2 by dave however this did not fix my issue.
I have been exploring this further - it would appear it is a client side issue. I get high (20%) usage when opening the course outline in studio. If I watch the CPU usage in task manager it stays like this for a while, before dropping to basically 0% for the tab that has studio running.
I wonder - is studio getting all content from the course and processing it in the background when opening the course outline, is this intended behaviour?