We ran some A/B tests where we intentionally slowed our site down (at the webserver page generation level) by set percentages (0%, 10%, 25%, and 33% IIRC). Those would be against a baseline page generation time of around 2000ms, so the slowdowns were not small.
All tests showed a drop in traffic, conversion, and average session value indistinguishable from zero. Our hypothesis is that our site (where a converting visitor is on average designing and ordering a custom product) is one where the difference between 30 minutes and 31 minutes is not meaningful and a lot of the time is spent in the web-based document editor. It would not generalize to a "give me a quick search result", "sell me an item from a warehouse shelf", or "let me argue with someone on a forum".
This was of course disappointing, because we ran this test as a sanity check after having done an extensive project to improve the page speed across the main conversion funnel. The project was technically successful, but a business zero (probably for the same hypothesis as above), so we decided to test in the other direction, because that's easier to implement than to force faster pages.