Poor Performance on newest iMac

Mac Technical Support
Have been on my 2019 iMac (3.6 i9, Radeon Pro Vega 48, 1Tb SSD, 40Gb) for about a week now, and it's been a huge upgrade for all my games except Diablo III. I first noticed a big performance drop a few months back when I switched from Sierra to High Sierra, and under Mojave those issues remain. The frame rate will randomly drop by half or briefly freeze altogether, causing a noticeable lag or stutter. No amount of fiddling around with game settings has had any effect on this (apart from setting sound channels to 128, which will absolutely tank FPS for some reason.) All these issues become noticeably worse in multiplayer.

This game also causes my GPU to run full blast at all times, which seems weird given its age and (presumably) undemanding nature. I have tried running it at lower resolution than the native 5k, but this produces undesirable video artifacts in some haze/glow effects (most readily reproducible in Belial's death animation) so I am wondering if there may be an issue with the video card itself.

Seriously though, why is a five-year-old game running so poorly on a brand new Mac? WoW is running great, for what it's worth.
It's because the game is using the OpenGL API and you're on 10.14 (Mojave). Mojave's OpenGL performance is absolute junk. Moreso for nVidia hardware than AMD, but even AMD's latest GPUs aren't immune because Apple just hasn't updated OpenGL for over seven years now. Any game that hasn't transitioned to Metal is going to have very bad performance I'm afraid.

What's going to make this annoying for you is that Diablo 3 will likely never see an update to Metal because it is now a Legacy/Classic game and is essentially in maintenance mode. Because it isn't a priority, it isn't using Metal. Unless the game becomes unplayable due to OpenGL being fully removed as opposed to simply being deprecated as it is now, I wouldn't hold out for any improvements.

If you could run High Sierra (10.13.6) I'd advise doing so, but your machine is too new. It cannot downgrade to High Sierra from Mojave. This is one of those edge cases where a new machine means more headaches.

This also assumes you're running via SSD and not a HD. CASC enabled games will run poorly on APFS when run from a HD as opposed to an SSD. APFS is now the default filesystem used by MacOS / OS X and for the average user it is very difficult to properly go back to HFS+ without the aid of something like Carbon Copy Cloner and an external HD (to hold your OS clone while you reformat the internal drive).

I'm sorry I can't give you better news. This is primarily an Apple created problem, not Blizzard created. Any game using OpenGL, including emulators, will suffer greatly under Mojave.
Not surprised to hear it. As you said, backgrading to an earlier OS is not an option (though the FPS issues were just as present in High Sierra.) It's possible things may have initially worsened with the most recent D3 patch, though I haven't been keeping track.

I just hope that if Starcraft II can warrant an option for Metal (however low-effort) that D3 might some day as well. As it is, it's borderline unplayable and that is a bummer for any number of reasons.
04/20/2019 11:26 PMPosted by TheTias
Any game using OpenGL, including emulators, will suffer greatly under Mojave.


I can't see any reason for OpenGL performance being worse in Mojave than High Sierra. If that really is the case, is there any explanation for this to be found?
If OpenGL is the problem, I wonder why the game plays fine for me on a 2010 Mac Pro. Am running SSD's, obviously APFS format & latest Mojave thanks to a GPU upgrade (Radeon RX 580).

It would seem something intrinsic to newer macs that is the issue.
I can attest that Diablo 3 runs really fast (sustained 60fps with Vsync on at 2560x1440) on my 2018 MacBook Pro with Vega 20. But this machine is only supported on Mojave so not sure about High Sierra OpenGL performance.
I easily run 60 fps at 2560 x 1440 on the same iMac you are referring to. It was choppy until I turned on the Low FX option.
06/15/2019 07:03 PMPosted by Oni
If OpenGL is the problem, I wonder why the game plays fine for me on a 2010 Mac Pro. Am running SSD's, obviously APFS format & latest Mojave thanks to a GPU upgrade (Radeon RX 580).

It would seem something intrinsic to newer macs that is the issue.


You're running AMD hardware. That's why it's not really that bad. OpenGL is still a bit fubared on Mojave for AMD hardware though, just nowhere near as bad as with nVidia hardware currently (Apple refuses to let nVidia release drivers to alleviate the problems).

On the I/O side of the equation, your APFS drives will eventually show some pretty gnarly slowing the longer you go without simply reinstalling the game (it has to do with b-tree extents nodes when files are changed in any way that add up over time). Anyone running APFS from a platter based HD is going to be banging their head against the wall sooner rather than later at the poor performance. And that's just the OS. Gets worse with the games that are patched frequently.
06/27/2019 05:18 PMPosted by TheTias

You're running AMD hardware. That's why it's not really that bad. OpenGL is still a bit fubared on Mojave for AMD hardware though, just nowhere near as bad as with nVidia hardware currently (Apple refuses to let nVidia release drivers to alleviate the problems).

On the I/O side of the equation, your APFS drives will eventually show some pretty gnarly slowing the longer you go without simply reinstalling the game (it has to do with b-tree extents nodes when files are changed in any way that add up over time). Anyone running APFS from a platter based HD is going to be banging their head against the wall sooner rather than later at the poor performance. And that's just the OS. Gets worse with the games that are patched frequently.


Thanx for the reply. Must admit some previous patches required deleting files or reinstalling game or Battle.net app. All good for the last handful of updates tho.

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