It's still not a good idea to put every reachable hostlist to Gravity's table and then have millions of blocking domains. But for testing purposes I had 62 hostlist, resulting in more than 1.2 million domains for Gravity for days.
It turns out that every action at the UI, that forces a FTL-restart (adding/deleting an entry at regex list) seems to fit instantly. But it does not. On a RPi3B the FTL-restart takes 25 seconds under these conditions. The RPi4B is much faster, but needs 15 seconds anyway.
I think it will be a good idea to have a large visible indicator at the UI showing the user that theres time to wait until FTL is up again. Maybe the state of the FTL-process can be achieved via PHP directly or via a pid-file.
Could be important when the actions in the group management will force FTL restarts after changing something, so such situations occure more often than earlier.