The spell checker in Calc only checks the on screen part of wrong spelled words. In principle no issue with that. However if you start scrolling, the spell checker 'restarts' checking everything on screen. So also the already checked part. Nothing is cached..
So spell checker is doing same stuff multiple times.. At the same time causes the restart the spell checker repaints of the red underlining.. which means shaping text over and over. And shaping text isn't cheap anymore (since Harfbuzz).
Which - both together - creates an massive CPU load while scrolling moving text
Steps to Reproduce:
1. Open the attached file
2. Automatic spell check enabled. Scroll up down & monitor CPU usage
15-25% depending on scroll speed etc
Can be improved
User Profile Reset: No
Version: 18.104.22.168.alpha0+ (x64)
Build ID: 59939d2490726336546c7ad05082d23031074e12
CPU threads: 4; OS: Windows 6.3 Build 9600; UI render: default; VCL: win
Locale: nl-NL (nl_NL); UI: en-US