panel unhide slow after virtual desktop change

[David L]

My 16.10 kubuntu system is currently in state where hidden panels
unhide slowly (>5 seconds and possibly proportional to the number of
desktops I change through... I saw a 30 second delay once) after I
change virtual desktops.  It's currently reproducible but I suspect a
logout and login would fix it because I'm pretty sure I would have
noticed this behavior before this if it happened all of the time.  The
CPU load of Xorg and plasmashell are highish during this period.

3252 root      20   0  289212 164096  93696 S  75.8  1.0  41:42.34 Xorg
3629 dgl       20   0 6576264 873620 269468 R  66.2  5.3  15:13.88
plasmashell

The desktops that I switched from/to were relatively empty.

There were some errors in .xsession-errors that may or may not be
related:
QXcbConnection: XCB error: 2 (BadValue), sequence: 15463, resource id:
138412037, major code: 142 (Unknown), minor code: 3
QXcbConnection: XCB error: 2 (BadValue), sequence: 15464, resource id:
140509189, major code: 142 (Unknown), minor code: 3
QXcbConnection: XCB error: 2 (BadValue), sequence: 15465, resource id:
83886085, major code: 142 (Unknown), minor code: 3
QXcbConnection: XCB error: 2 (BadValue), sequence: 15466, resource id:
12582917, major code: 142 (Unknown), minor code: 3
QXcbConnection: XCB error: 2 (BadValue), sequence: 15467, resource id:
109051909, major code: 142 (Unknown), minor code: 3
QXcbConnection: XCB error: 3 (BadWindow), sequence: 48602, resource
id: 29802249, major code: 3 (GetWindowAttributes), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 49890, resource
id: 29802465, major code: 3 (GetWindowAttributes), minor code: 0

Any ideas why it is in this state or how to get out of it without
restarting this session?

Thanks,

       Dave