I think an underappreciated aspect is that we previously amortized the reading and comprehending over the time we spent designing and writing. I still find myself looking at changes expecting that I can review it in a few minutes when I am in effect coming in cold. That part of the job has changed, but fwiw it isn't so different than the experience I had reviewing another team's changes when I didn't know their services.

I have found the right harness helps, if you have a setup that gives you memory, specs and a graph-based understanding (that part is key) you can be very productive. For me, that's because being able to understand the big picture helps, the challenge of reading the code is always there but starting with 'this method/class/etc does X, so that means this is doing Y..." makes it easier and lets you come in warmer.