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:

• start the game
• place a "Slime Pit" or "Angry Nimbus"
• press e for inspect and check out their "Rate"
• upgrade them and inspect them again and check out the "Rate"
• notice the tower is attacking slower now.

Expected result:

On upgrade both towers are getting faster attack rates.