Trying on that very same 2.10.34 on Linux on a regular PC and using Windows > Dockable Dialogs > Dashboard to monitor CPU usage the CPU never goes above 3% even with a large (200px) softened brush at usual spacing (10%). Things are a little harder with a 1% spacing where I reach around 10% CPU.
Also, how much RAM have you got, and how much memory, cache and swap is Gimp using (see also the Dashboard dialog).