This is an interesting thread to read, I've not played/run 5.5 so haven't seen weapon mastery in play but I always felt they didn't quite work as is but otherwise looked like a good addition (I was planning on some house rules if I did move, mostly that warriors learn a weapon mastery keyword rather than a specific weapon on the off chance that they switch to a new weapon with the same keyword before able to swap out the proficiency).
I'd say the comparison with cantrips isn't quite applicable in that, particularly from 5th level onwards, there is still only a single attack roll or saving throw for a cantrip rather than an attack and then a saving throw. Once you get up in levels and additional attacks come online, I can see that slowing things down a little. Again, I've never run 5.5 so interesting to hear that many have had a negative response to weapon mastery, though the poll looks roughly even between the yeas and nays.