I also have many tabs (that's why I could quickly recall and find the post). Restoring session takes a while (much more than 15 seconds). I measure this time by looking on the CPU consumption. Only once it drops to near zero I consider session completely restored.
I just timed it for accuracy. When launched alone, on my M2 MBA, it's about 18 seconds to full draw of visible tab list, and 29 seconds to snappy interactivity. I didn't check CPU utilization.
Usually, when I launch this 20+Kt profile, I also launch 2-3 other profiles simultaneously (work 2Kt, personal/misc 3Kt, sometimes commerce 400t). I've noticed that they each peg a core while launching, but this is the only one that isn't ready quickly.
I also have many tabs (that's why I could quickly recall and find the post). Restoring session takes a while (much more than 15 seconds). I measure this time by looking on the CPU consumption. Only once it drops to near zero I consider session completely restored.