An example of the sluggish interface is that from I click the loupe icon to search (around 00:02 on my stopwatch) it takes until 00:09 to show the search field - And I then paste the search string in - because typing is also slow.
Thank you for reporting this and providing a video! We will be improving the search speed in future releases.
Not sure if the file you're importing is a complete Bootstrap theme, but if it is, the recommended way is to import it as a Custom Theme from the Settings dialog. Otherwise you end up with two Bootstrap files at the same time.