We can't know the answer. What you are describing is a GPU hang and GPU hangs are a whole class of bugs, it pretty much means "the hardware was misused and now it doesn't know how to move forward, so we reset it" (and the reset is why you get the 5s freeze, your graphics card was essentially rebooted). When this happens, i915.ko creates a nice file in your file system that you can use to create a bug report. Please check dmesg and do so.
Thanks for the overview. I know HN isn't the place for this but just for the record: there are no other i915 messages in dmesg (certainly nothing about a file with debug info).
This is on Ubuntu 19.10, kernel 5.3.0, and it started happening after a minor update, so I figured it would probably get fixed in another minor update. That didn't happen, so then I thought I'd better update to a newer kernel before reporting anything. Then 20.04 was almost here, so I decided to wait for that. Hopefully I'll get around to that soon and discover my particular bug has been fixed, otherwise I'll try to report it.
Seeing the hang message without anything else is surprising to me. I don't know why you're seeing that.
You can try kernel-ppa [0] to see if it's fixed in newer kernels, it's very simple and you don't need to compile anything. I would recommend you to test 5.7 from there, and if that doesn't work you can try drm-intel-nightly or drm-tip: those are the upstream graphics trees. If you report a bug, the first thing the devs are going to ask you is test these trees, so it might be worth trying before you even open a bug report.
Just a quick followup: I installed 5.7 (still on Ubuntu 19.10) and no hangs yet.
I got a bunch of warnings about missing firmware so I manually downloaded some i915 firmware files from the linux-firmware repo. I assume that's just an Ubuntu packaging quirk. I'm not sure if they were actually needed or not.
I had the same problem on Ubuntu 18.04, terrible lag spikes and similar messages regarding i915. After installing the least new kernel (5.3.1), it solved the issue: not one hang since.
