Well, I would first do a
fetch check and take it’s time. If that fails quickly, I’d see that as an Indicator for a blocked URL.
Only for those URLs, I would open a single or a couple of tabs and consecutively test all questionable URLs in those. Also, the page load can be canceled as soon as
If you want to hide the loading tabs, you can open a popup window in the background. It is actually possible to open additional tabs in those, which can’t be accessed by the user (a bug, I guess).
I doubt there are (m)any users with 5000 blocked bookmarks.