Preview Delays

I'm on a 2015 iMac with 16gb memory and 4ghz processor. When I open a folder or collection in grid mode and start scrolling down my previews do not display right away, it takes up to 30 seconds, sometimes longer for them to display. Essentially I scroll then have to wait for CO to catch up. I am not referring to regenerating previews, just the initial displaying of them. I've confirmed that previews are not regenerating via the activities window.

I have tried with and without hardware acceleration.

I was wondering if any one else has noticed this and what other steps I can look at to improve performance.

Thanks.