just upgraded iMac late 2015 ram from 12 GB to 28 GB and it became too slow to use

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

Solution 1:

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.

Solution 2:

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.