This is strange... I'd say it's because the attack speed progression is geometric, so the more you have, the harder it gets to get faster, but if you're saying the uncommon upgrades are working, then it must be a bug :(
Yeah, the first time I actually tested it (at first I didn't notice) I had ~1.5s/attack. After 5 common upgrades with 3 tomes I still had ~1.5s/attack. It should have been 37.5% faster for each at that point, which should have dropped it to about 0.14s/attack. And as soon as I got the uncommon one it went to ~1.1s/attack
Edit: Just to confirm for you, the 2.3 patch does not appear to have changed this bug. 0-1 tome still operates as it should, 2+ tomes cause common attack speed upgrades to have no effect, uncommon versions still work properly.
I did not expect it to have changed, just thought I should let you know after the patch as I am aware of how strangely code can behave sometimes...