The usleep here isn't one that was intended to be taken frequently (and it's taken in a loop, so a too short sleep is ok). As it was introduced, it was intended to address a very short race that would have been very expensive to avoid altogether. Most of the waiting is intended to be done by waiting for a "transaction lock".

Unfortunately somebody decided that transaction locks don't need to be maintained when running as a hot standby. Which turns that code into a loop around the usleep...

(I do agree that sleep loops suck and almost always are a bad idea)