Editing slows down over time

Working on a normal timeline, some Red material, some ProRes and some H265.
Editing is going fine for a few hours of work, but then it starts to slow down, any command or change in the timeline takes 1-2 seconds to happen, regardless of what type of material or whatever.

There is no reason for this slowdown as it works perfectly well for a few hours. It just slows down all of a sudden. I have no cache or proxies in this, so there's no change in material or drive location.

What's going on here?

Working on a normal timeline

Out of curiosity, what's an 'abnormal' timeline?

What's going on here?

Post your system specs, including storage.

Working on a normal timeline

Out of curiosity, what's an 'abnormal' timeline?

Well, basically a ton of VFX, transitions, opacities, layers and so on. There's none of that, just pure hard cuts.

What's going on here?

Post your system specs, including storage.

i9-7960X
64GB RAM
RTX 2080TI
18TB RAID0 (3 x 7200RPM)

But I've never had any problems before, neither with editing in Premiere or in Resolve.

just pure hard cuts.

Ahhh. I think I'd call that more of a 'simple' timeline.

OK, that RAID is internal or external?
If internal, is it hardware or software? (You have a RAID controller installed?)