Thanks for the advice.

I'll go down the route of a batch file and setting the priority to /low of /below normal

The PC is pretty powerful with 8 cores, 40Gb RAM and SSD throughout. The blockchain is on its own disk.

So, I have the Windows startup command

D:\temp\BTC.bat

which says:

@echo off
set
start "Bitcoin Core" /LOW "C:\Program Files\Bitcoin\bitcoin-qt.exe" -min -chain=main

It seems to be OK so far