Fun fact:
Almost one month after the re-launch of my forum, I did another spam accounts cleanup since more than 8000 remained even though I did massive cleanups at various stage of my migration.
On 180000 accounts, only 13500 were legit, active accounts. 
166500 accounts were spammers or old accounts that never had posted anything!
Unlike Discourse, vBulletin doesn’t automatically clean up unusued accounts.
It was the same on my migrated phpBB forum, on which the number of accounts decreased from 20000 to 3000 with the automatic Discourse cleanup only (no manual cleanup from me).
And thanks to @Johani, I replaced my GIF loader by a proper CSS animation.