Implementing Speed into a Rotation

I'm making a 2D, top down tank game and the turret needs to rotate towards the enemy tanks. I've got a script that rotates the turret but I can't figure out how to implement speed into the rotation so it doesn't turn at lightning speed. Here's the script that I've currently got,

I'm pretty new (not a total beginner though) so please don't use complex terms.

Thanks in advance