- This systems RAM is soldered! So you are stuck with the 8 GB it offers
- If you have a HDD model you won’t have the PCIe blade SSD connector and its support logic so that rules out any performance improvements by adding a faster SSD drive
- The CPU is also soldered down so you can’t upgrade it
- This system can’t support an internal 4k display either!
Basically, it is the least upgradable system Apple made!
So the only thing you can improve is swapping out your SATA III (6.0 Gb/s) HDD to a SSD. Here’s the guide to do that iMac Intel 21.5インチ EMC 2805ハードドライブの交換. If you have a PCIe blade SSD only system you could add in a HDD or a second SSD.
If you happen to have gotten a Fusion Drives system then you could replace the impish 128 GB blade SSD to a more sizable blade SSD (500 GB to even a 1 TB) as well as breaking the Fusion set so you end up with a true dual drive setup which will get you even better performance! iMac Intel 21.5インチ EMC 2805ブレードSSDの交換
Frankly, I think finding a used Mid 2017 21.5” would be a better and cheaper direction iMac18,2 - 21.5” 4K iMac Core i5 3.0/3.4 GHz or a i7 3.6 GHz as you are also looking for a 4K system.