• OS: 10.14.6 (18G4032)
• Browser: Chrome Version 80.0.3987.149 (Official Build) (64-bit)

Steps

• Visit https://office.wikimedia.org/wiki/Contact_list?veaction=editsource for example
• Start editing
• After a bit the whole browser becomes very laggy, editing that page is very hard and laptop fans start blowing strong

Expected

• Editing is smooth and the browser doesn't lag

Other details

• When opening "Activity Monitor" there is a process called AppleSpell using 70% CPU, and Chrome uses 30% more
• If you don't edit for a while, the process finishes its job and CPU usage goes back to "normal"
• If you edit anything else the situation starts again

This makes the edit source mode basically unusable.