Grimrock

I made a group with 2 spellcasters. Now I'm bit regretting it as I feel it takes always several mouse clicks to get any spell action completed. My front row fighter and rogue are nice and easy to play, just one click hit.

Is there something to make spellcasting more fluent, like having quick keys or so? Or am I better off making a new group?

The best thing you can do is queue up your runes in preparation. If that doesn't help enough all you can do is start a new group.

There are some pretty complex AutoHotKey scripts that can do it