Thanks - I ran that script for both different service login accounts.

Both times the script gave identical results:

auth_scheme SQL

login_Name sa

Bizarrely too, I also did a SELECT * FROM MyTable when logged in under the domain account, and this time it was fast, that is - the expected time. Must've been something playing silly buggers on my PC.

I'll keep switching between the two over the next few days and see what happens.