SUMMARY
Resizing the window by dragging from window borders is extremely choppy, makes UI elements flicker and jump around all over the place, and causes plasma-systemmonitor CPU use to increase proportional to drag velocity. ~60% of one core (as self-reported by systemmonitor) is easily achievable. 
On a 10900K @ 5GHz, using this much CPU time to resize a window is completely ridiculous.

STEPS TO REPRODUCE
1. Have eyes.
2. Resize the plasma-systemmonitor window.
3. Observe.

OBSERVED RESULT
Horrible UI performance, unacceptable CPU utilisation.

EXPECTED RESULT
plasma-systemmonitor performs as well as ksysguard, and doesn't create the very problem "system monitor" applications are used to observe/diagnose.

SOFTWARE/OS VERSIONS
Operating System: Gentoo Linux 2.15
KDE Plasma Version: 6.1.5
KDE Frameworks Version: 6.5.0
Qt Version: 6.7.2
Kernel Version: 6.6.51-gentoo-dist (64-bit)
Graphics Platform: Either


ADDITIONAL INFORMATION
Other "modern" plasma applications (e.g. systemsettings) also have nasty jitter and eat excessive CPU cycles when drag-resizing their windows (i.e. this is probably a toolkit problem), but plasma-systemmonitor is by far the worst offender.
This is present in both wayland and X11 sessions.