The old adage is if a tool wears out too fast, it's too much rpm. If a tool breaks, it's too much chipload.
Running a tool at higher than recommended surface speed wears it out too fast principally by making it too hot, which softens and dulls the edge.
If you're going to adjust something for conservatism, better to slow the rpm than the feedrate lest you make the tool start rubbing from too little chipload. Rubbing will also wear out a tool prematurely, which just goes to show those old adages don't tell the whole story.