Portable Firefox takes a long time to load a website – sometimes

Hello,
I regularly use Firefox Portable (currently v. 89) and I am confronted with this issue.

If I quit Firefox Portable normally and restart it, it loads in a decent amount of time (maybe 10 seconds before it loads a page). Same behaviour on the desktop version of Firefox, by the way. Likewise if I close my session but don't turn off my computer.

However, if Firefox Portable crashes OR if I turn off my computer, OR if I disconnect my portable hard drive and connect it to another computer and then restarts Firefox, then it opens at about the normal speed, but then it takes 3 to 4 minutes to even load the first page. And this is the case whether I close Firefox Portable normally (I usually close tabs before I quit) or whether it crashes.
And if I decide I am fed up and quit again Firefox and restart it so there would not be any session to recover, it still takes a long long time. Restarting in safe more doesn't make a difference.

On top of that, it seems to be a problem specific to the Portable version. Crashing the desktop version of Firefox or restarting it just after I shut down and restart the computer doesn't create that kind of problem.

My questions:
– Is there a bug?
– Is there a setting that I should look for and change?

Thanks.