Bug 460348

SUMMARY
I updated my Debian Sid to kde 5.26 yesterday. After restarting today, I noticed that the UI is getting sluggish from time to time or even hangs for a while. In htop, it looks like plasmashell is always lingering at 25-30% of CPU.

The machine is Debian Sid, on X11, with a nvidia 2080 ti, and latest 510.0 nvidia drivers from the debian sid repo.


STEPS TO REPRODUCE
1. upgrade debian sid to kde 5.26
2. restart
3. log in to kde X11
4. run htop


OBSERVED RESULT
plasmashell takes up 35-30% of cpu, UI is sluggish and hangs

EXPECTED RESULT
pasmashell is somewhere down in the noise in CPU usage and spikes a bit up when more UI intense stuff happens.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux
KDE Plasma Version: 5.26.0
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6
Kernel Version: 5.19.0-2-amd64 (64-bit)
Graphics Platform: X11
Processors: 48 × AMD Ryzen Threadripper 3960X 24-Core Processor
Memory: 31.2 GiB of RAM
Graphics Processor: NVIDIA GeForce RTX 2080 Ti/PCIe/SSE2
Manufacturer: ASUS


ADDITIONAL INFORMATION

Having stumbled upon bug #460248, I checked the output of 
$ sudo perf top -K -p $(pidof plasmashell)

Samples: 10K of event 'cycles', 4000 Hz, Event count (approx.): 2988952206 lost: 0/0 drop: 0/0
Overhead  Shared Object                  Symbol
  15.59%  libKSysGuardSensors.so.5.26.0  [.] KSysGuard::SensorDataModel::data
   5.20%  libQt5Core.so.5.15.6           [.] operator<
   4.78%  libQt5Core.so.5.15.6           [.] QVariant::QVariant
   3.49%  libQt5Core.so.5.15.6           [.] QVariant::toString
   2.87%  libQt5Core.so.5.15.6           [.] operator==
   2.53%  libQt5Core.so.5.15.6           [.] 0x000000000014af6d
   2.43%  libKSysGuardSensors.so.5.26.0  [.] 0x0000000000014910
   1.75%  libKSysGuardSensors.so.5.26.0  [.] 0x00000000000148c6
   1.70%  libKSysGuardSensors.so.5.26.0  [.] KSysGuard::SensorDataModel::rowCount
   1.68%  libQt5Core.so.5.15.6           [.] QVariant::~QVariant
   1.54%  libQt5Core.so.5.15.6           [.] QVariant::toDouble
   1.43%  libQt5Core.so.5.15.6           [.] 0x000000000014af30

I'm not familiar with perf, but it looks like the ksysguard sensor reading is taking time?
I have three sensors active in my top bar:
- Individual core usage
- Total CPU usage
- Memory usage

If I get rid of these, the behaviour is the same. But if I reboot after removing them, plasmashell cpu usage is normal.
Adding one of those sensor widgets again bumps plasmashell cpu usage up again.