This will make it so that (on systems with timerfd) the runtime will no longer have to wait for the next runtime tick before exiting. This saves on average 5 ms per program run, and up to 10 ms for programs with a very short runtime. Not much for server programs with an expected lifetime of days/weeks, but potentially quite nice for CLI programs like hledger or for programs that invoke many other Haskell programs.
I accidentally discovered this when trying to optimize an Advent of Code problem and couldn't get it below 10 ms no matter what I tried. Eventually I discovered that "hello world" also took 10 ms so I assumed it was just runtime startup costs. Decided to look anyway and found out it was actually runtime shutdown costs, where the runtime would `join` the rts "ticker" thread. The ticker thread would only check for a shutdown flag once every loop, so 10 ms with the default settings. Now it can be shut down immediately, which makes small programs much faster.