I have just discovered (one of?) the reasons plasmashell was eating so much cpu and memory on my system. I have dual monitors and in circa FC20 I could get a panel on both screens then something happened (and I suspect I tried apparently unsuccessfully to restore a panel to one of the screens) and pretty well from then on the plasmashell problem grew. After looking on the kde lists for a clue as to what the problem might be (I notice its a "well known bug") I espied recently a comment about someone deleting a panel and finding there was another panel underneath!!!! So I deleted the panel on the screen that had one - lo' it was "still there" - I deleted that ..... for about 10 of them!!!!! The last one disappeared - but I could restore it (undo the delete) Now with just one panel my kde desktop has finally reverted to sanity after about 6 months! I will now try to work out exactly what happened and when it happened.
Anyone else seen this?