Noticing @rosenrot and I had slowdowns with around the same number of open tabs. Maybe some data structures overflowing some cache and thrashing after a certain number of tabs are active?
I noticed a somewhat non-linear slow when going from tabs in the low-20s to high-20s the other day (was running Version 0.99.127.3.2-rc). Didn't know why the system felt so sluggish at first so I started through my "todo" tabs in preperation to reboot and only noticed after I worked through a few tabs that the system felt so much faster. Unfortuantely didn't have a chance to capture more details in the moment so didn't report it.
FYI, these notes are from running on a MacBook Air with the i3-1000NG4 processor which has 512KB/core of L2 cache and 4MB of L3 shared across both cores.
I realize this system has a low-end processor so trying to focus on issues that might be applicable to other people -- either when Orion runs away with the CPU or non-linear slowdowns.