SOLVED!
Bloody SMC reset on my Mac.
I did a Geekbench test; my machine was running at a third of its potential.
Now all good again.
Thanks for all help.
Adam
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Great that you figured it out! 
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So this was on 10.12? Sierra has been a real mess on many fronts.
Can you post your Mac model just for reference? Thanks
Details of my machine attached.
For info:
Ableton crashed. Following crash, everything seemed fine other than poor performance.
SMS reset solved everything.
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