good question. i've seen frametime been brought up a few times as the true measure of this issue but i don't understand why. frametime is measured as as inverse (in ms) of the framerate to get the millisecond per frame (ms/f) i.e
Code:
ms/s % frame/s = ms/s x s/frame = ms/frame
so for a fps of 20:
frametime = 1000ms/s x 1s/20frames
= 50 ms/frame
likewise fps 60 = 1000ms/s x 1s/60frames
= 16.7ms/frame
as you can see, those values are exactly on point.
EDIT: i totally misunderstood your question. sorry for the elaborate reply. the framerate spikes that you see are at least PARTIALLY explained by different things popping up on the screen or menu access. not all of them mind you, but when i access a menu, talk to an npc, trigger a cutscene and a number of other actions, the framerate does that spike.
you understand that the VRAM itself is not actually a different speed right? the ram chip that's being accessed is of the exact same quality as the rest of the memory on the card. the method of accessing is what's at issue here. ANY access into that partition should show at least SOME notable decrease in performance if your theory of a 15-20fps drop held any water, which it doesn't.
now that we cleared that up, i played another hour or so of SoM and ran into the same issue as before with the weird stutters. i THINK it's caused by the card swapping out memory to try to stay under the 3.5GiB mark, similar to what would happen on a functioning card if you maxed out it's memory (say a 980 with the VRAM pegged at 4096MiB). to show it a bit more clearly, i zoomed in the mem usage portion of the graph. the dips fall fairly well in line with the times of my experience of the graphical anomalies. i had the afterburner OSD running so i was able to see it happening in realtime.
note: not all the dips in mem usage are the cause of the card, some happened when i triggered cutscenes while others happened when i accessed the menu. in fact, i think it was the little dips more than the big ones that occured at the same time as my gfx issues. edit: i should also note that this just goes to further show that the 970 is in essence a 3.5GiB card or at least wants to be one. lol.