Bug 441140

SUMMARY
When changing virtual desktops, Plasma hangs for several seconds.

STEPS TO REPRODUCE
1. Have lots of windows open on origin/destination virtual desktops
2. Switch virtual desktops

OBSERVED RESULT
Plasma is frozen for ~8 seconds

EXPECTED RESULT
Plasma continues working normally

perf report during virtual desktop switching a few times:
Samples: 148K of event 'cycles:u', Event count (approx.): 129861648422
Overhead  Command          Shared Object                                    Symbol
  74.48%  plasmashell      libxcb.so.1.1.0                                  [.] _xcb_map_remove
   5.27%  plasmashell      libtaskmanager.so.5.22.0                         [.] 0x0000000000067ed0
   2.11%  plasmashell      libtaskmanager.so.5.22.0                         [.] 0x0000000000067ec4
   1.13%  plasmashell      libQt5Core.so.5.15.2                             [.] QIdentityProxyModel::index
   1.08%  plasmashell      libQt5Core.so.5.15.2                             [.] QIdentityProxyModel::rowCount
   0.74%  plasmashell      libQt5Core.so.5.15.2                             [.] (anonymous namespace)::convert
   0.65%  plasmashell      libQt5Core.so.5.15.2                             [.] QAbstractProxyModel::data