I've been noticing Xorg spikes - especially when watching Youtube - on an older machine of mine. I thought it might be the distro (the machine doesn't run Solus) and was contemplating a switch to another one, but perhaps it's a distro agnostic problem and Solus is also affected.
Anyhow, in my case it seems not using a compositor makes the problem practically imperceptible, so I run Openbox on that machine with no compton. With compton running or under GNOME I see spikes very similar to what you are showing, except on an nvidia card. You can try a lighter WM as a workaround until a real fix appears.