changed something and slowed things down

I was mucking about with various settings (smart, I know) and seem to have changed something that has caused the main viewport to become very slow.
I don't mean that manipulating it has become slower, I mean the length of time it takes before the iRay 'preview' within the viewport becomes nice and smooth has extended. I specifically mean when using the iRay view too, rather than texture shaded for example.
Has anyone got any ideas on which settings affect the speed of the preview render in the viewport when using the iRay viewport option?
Unfortunately I went to town on settings and have messed around in:
Preferences > Interface
Draw Settings
Render Settings
Both Render and Draw are set to 'Photoreal', that definitely hasn't changed. I've set Render settings back to default now as there's a handy button for that, I was hoping that would restore my viewport to it's former glory but it hasn't.
Thank you in advance, any assistance is much appreciated.
Comments
I think reloading your layout (WindowWorkspace>Select Layout) will reset Draw Settings. Before that you could check Edit>Preferences>Interface>Display Optimisation is set to best, but I don't believe that will affect the Iray preview mode.
Thanks Richard.
Neither of those seem to make any difference, are there any other settings/options that you can think of that would affect the time it takes for the viewport preview to render?
@supernoob
Do you have an Nvidia GPU? If so, have you possibly disabled it in the Render Settings tab?
I just checked, it's not that either