I’m realizing I didn’t really address the question posed in the title, since it really needs to consider how the initial cards are added (most of my 4,600 were added to my system large groups as prebuilt decks). It would probably be a different story if I started with 0 cards and added 4 new cards a day. In that scenario, it might be possible to increase the load without increasing the daily review time (4,600 cards would take over 3 years at 4 cards per day).