The "Angry Nimbus" and the "Slime Pit" are attacking less the more you upgrade them. If you check on their inspect you see that the "Rate" is getting bigger, meaning they attack less each upgrade.
For the Angry Nimbus and Slime Pit the speed is reduced on the first 2 upgrades and the last 2 did nothing to speed.
Steps to reproduce:
On upgrade both towers are getting faster attack rates.