It turns out that there isn't one big break, there are many small cracks that have been patched. I was reading about 13% performance increase in benchmarks, which seems significant.

Ah, here it is: https://www.phoronix.com/review/debian-13-benchmarks

If you have an nvidia card, which is very common and can't always be avoided, there is significant benefit in newer drivers, for one

More like "If it isn't as good as it could be, don't do anything to improve it."

So I guess you're still using a PSTN modem ?