After having upgraded to kubuntu 15.04, plasmashell started eating ~110% of CPU time.
I have a three monitor setup (radeon 5750 with oss drivers) and didn't have such a problem with kde4. Anyone having same issue?

I'm going to regret to have upgraded to 15.04..

Problem was plasma-nm continuously trying to fire up a bridged connection. After clicking "Disconnect" on the connection, plasmashell lowered its cpu usage to a more pleasant 4%.

I found plasma-nm both unable to configure new bridge connections and to manage an already configured one.

I don't know about network bridges.
The CPU problem you are having is a known bug in the intel driver triggered by certain animations in plasma, not only plasma-nm. I have the same issue and currently there seems to be no way to fix it. I just avoid having things trying to connect all the time.