What's your hardware? and how long did you wait since upgrade?
I did experience the same/similar behaviour on my Pi2B/1GB, see Hardware recommendation for v6? - it did normalize after about 6hours and now I'm back on the Pi2B pi-hole.
edit: I think it was, because my FTL database is about 4,5 GB in size. Perhaps check yours?
pi@pihole:~ $ ls -lah /etc/pihole/pihole-FTL.db
-rw-r----- 1 pihole pihole 4,3G 20. Feb 20:17 /etc/pihole/pihole-FTL.db
see also PiHole 6 Slow start-up with large gravity lists - #11 by grdnwsl