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?
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?