I've been doing some testing and I have to wonder if they aren't silently reducing the amount you gain from farming within a certain time frame.
Last week when I measured a solo +4x8 run of this farm map (2915), I was getting about 7 mil per run. Today I'm getting 5.5 mil per run. You're saying you're getting 15-20 mil when it takes you an hour to clear, and my runs take me 5 to 6 minutes. I'm capping out around 60mil/hr
Seems to me like the exemplar double inf nerf isn't the only nerf that happened. I haven't gone back through old patch notes to confirm, but it's really starting to look like you're also capped at how much inf you can make per hour.