I think excluding threading was the big win

Yes, you want to stick with that one.

a switch to 5.10 from 5.8, saved about 30%

I think a switch back to 5.6 will also provide a further significant reduction (untested). You could even go further back, if you like - though the further back you go the more likely it becomes that you'll have to make amendments to the code you want to run.

Cheers,
Rob