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I had added an 8GB RAM along with existing 4GB when I purchased this iMac in 2016. For all this long it was running fine when lately I thought to add more RAM. So yesterday I added 2 more 8GB RAM modules and as I turned the system on it starts running to slow.

Apps which were running smooth earlier are now running too slow. On Civilization VI i can practically see every frame moving every second, which earlier was running very fine.

Old config RAM

  • Slot 1 from bottom 4GB - system default RAM
  • Slot 3 from bottom 8GB - Hyper Kingston 1867 DDR3 RAM SODIMM

Now added RAM

  • Slot 2 from bottom 8GB - Hyper Kingston 1867 DDR3 RAM SODIMM
  • Slot 4 from bottom 8GB - Hyper Kingston 1867 DDR3 RAM SODIMM
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    Why would adding RAM change a game? Seems your system not only didn’t need it, but has more things it’s hanging on to. If you remove the memory, does the performance return? – bmike Mar 2 at 15:33
  • Does removal speed it back up? – John Keates Mar 8 at 21:16
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You should have matching DIMMS in order to maximize performance / dual channel functionality. Also note that RAM is going to operate at the lowest speed of all the DIMMS, which is another reason why they should all match.

That being said, it shouldn't be operating that slowly. I would start with removing the original 4 GB stick, possibly doing a PRAM reset, and maybe shuffling around which slots they're installed in, since the problem only starting showing when you used the two vacant slots with the new RAM, if I'm reading correctly. Also confirm that the system is recognizing all the memory you installed.

Finally, it's possible you got bad/defective memory. You can also try running diagnostics to rule out logic board issues etc.

  • Are you able to make a benchmark and measure matching / non-matching DIMM? It was a thing back in Core2 Intel chip and logic board days, but for the last 5 years this has been a rounding error or zero measurable change. – bmike Mar 2 at 15:32
  • It depends on what you're doing, but having more memory bandwidth in general shouldn't hurt. Gaming and general purpose usage isn't going to be affected much by this, but simulations, compiling/developing, etc could definitely. That being said whatever happened to the OP doesn't seem normal. Could be some weird incompatibility or quirk as I've seen Apple hardware be particularly finicky when it comes to RAM. – Hefewe1zen Mar 2 at 15:54
  • Agreed on older hardware. For me 2014 / 2015 is when all that went away. You’re right, something really strong is happening here. Also, my answer might be better off on a more general question. There’s going to be something on frame rate drastically dropping like a chrome tab or something just pegging the CPU and nothing about RAM or you’ve nailed it and it’s flakey RAM. We’ll need OP to triage a little more. – bmike Mar 2 at 16:02
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    @Hefewe1zen Here some benchmarks which show that the whole single vs. dual channel performance is simply irrelevant for consumers. Some HPC code is carefully enough written that you can see performance gains from more memory bandwidth, but in practice latency and most of all quantity is what matters for memory. (Compilers also don't improve noticeably either - at least not when I last played around with that for compiling HotSpot). – Voo Mar 2 at 18:57
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I wouldn’t add memory to a system unless and until the memory pressure runs into the yellow or worse. More parts means more chance of failure, less chance that good compression and swap happen.

In fact, we’re seeing the same thing on 2017 iMac and newer in production. We get Apple NVMe SSD and they are so fast, the stock 8 GB of RAM is faster in almost all cases than bumping up to 16 or 32 GB of RAM.

There are some very limited cases where you need more memory, but generally, you want to not run web browsers and other buggy / leaky code and adding more RAM just gives them more room to make a bigger mess before you clean them up.

Even Photoshop and Illustrator - crank their undo buffers down to an accurate size for the needs instead of giving them 8 GB of RAM for scratch so they keep around undo from 4 hours ago.

These days, the performance bottleneck is almost in order:

  1. The human
  2. The human not cleaning up software (think 50+ browser tabs, with all sorts of JavaScript trackers running in each tab, web apps, node apps, electron apps, etc..)
  3. Just bad software (things that take CPU in the background, constantly wait for network tracking and can’t work offline or communicate only when there’s something to do)
  4. Network slow
  5. Server slow (SMB / local servers)
  6. Storage slow (especially non SSD / non NVMe storage)
  7. Maybe now you might need a CPU bump or some more RAM - but maybe not, so measure before and after to change setup.
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    for me #4 would slide up to #2 most of the time... I've got a sketchy router – dwightk Mar 2 at 15:36
  • Yes - there are lots of people that can quash some or all of these. I was saying if you took 200 people that you meet on the street and they say “my mac is slow” - this list would be my experience. Sketchy network can really wreak havoc - specially inconsistent DNS and things you don’t expect to be checking the network (like an IDE / code editor - but little snitch will show those apps “chatting away” reporting on each thing you do in the editor) – bmike Mar 2 at 15:52
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    Well #2 is directly improved by having more memory available. #3 is also again improved by more memory usually. So I guess, by your own list, adding more RAM is the most useful thing one can do? (personally I'd put the "non SSD storage" part at the top of the list, but considering how cheap memory is these days, there's really no reason to skimp on it). – Voo Mar 2 at 19:02
  • I would disagree. If you have bad software, adding more ram so #2 and #3 are harder to fix when you don’t confront the bad software sooner. I see your point @voo for one specific mac - if you assume you can’t change the software you run, throwing more RAM delays the pain or the eventual restart. The problem with general advice is RAM is a choice on over half the hardware you make when you buy and people are buying too much IMO out of fear that they can’t have good software choices. – bmike Mar 2 at 20:05

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