Well first step is to optimize, parallelize and refactor so you do not have a single process that takes hours, but many separate ones you can run at once in a cluster.

If those get too expensive to run or you cannot speed them up them you have to do what Chromium does: run them post commit then bisect and revert any changes that break the tests. If things are truly broken you close the tree for a bit while you get the break reverted or fixed.

Also the system that is landing changes tests the optimistically in parallel assuming they will all succeed, so it does land a change only 30 minutes for example.