Why is my hard disk drive iMac so slow?
The performance of the iMac 8 gigabyte (8 GB) hard disk drive (HDD) iMac configuration is discussed fairly often here in the Apple Support Communities; both the iMac 21.5” and iMac 27” models.
Why is this iMac so slow?
A fast HDD does ~150 to ~200 I/O operations per second. A slow SSD does ~100,000 I/O operations per second. You don’t have a fast hard disk drive with this model iMac, either. Which means you’re waiting…
What are I/O operations? If you’re using Mail for instance, launching Mail.app performs I/O reading in the app itself and all of its code dependencies, reading your mail settings configuration data more I/O, reading in your credentials needed to access the mail server means I/O, reading or writing a message yet more I/O, receiving mail means message-processing rules and junk mail filtering data must be read in, many apps will involve a lot of I/O.
Main memory can be used to mask some of the slowness around accessing slower main storage; whether I/O accessing HDD or other main storage. This is called caching. macOS keeps some of the storage data cached in main memory, so that it doesn’t have to re-load that data from main storage.
This caching behavior is also why users and apps looking to create “free” memory is wasteful: you paid for the memory, you really want it to be used for apps or data or caching. Creating free (empty) memory is memory not being used. Wasted.
Buying a fast processor with too little memory or with slow storage means you now have a fast processor that’s too often idling, waiting for data to move around. This purchase of a fast processor is great for the folks selling the processor, and not so great for your computer’s performance. Why? Computers are systems, and the performance of the system is only as fast as its slowest component. A 3.2 GHz processor that has to wait for its data to be written to or read from a slow HDD gets… bored.
For the same reason why buying faster processors might not be as big a benefit as any of us might want, you might think “well, why not just have lots of main storage, and not this hierarchical-storage design? Mainly because faster storage is more expensive than slower storage, just as HDD storage is cheaper than SSD storage.
So…. Price? The iMac 8 GB HDD configuration was built for its low selling price, and not for its performance or any useful longevity or other features. low selling price. This means compromises around the capabilities and performance. Recent macOS releases and bigger apps have meant that main memory is filled or over-filled, that data then needing to be written to or read from the HDD, and that HDD was slow to start with.
What to do, short of iMac replacement?
Internal iMac 21.5” and iMac 27” upgrades are expensive and difficult, where those upgrades are even possible. The most cost-effective performance upgrade to the iMac 8 GB / HDD configuration is to migrate to an external Thunderbolt or USB 3 SSD. Here is how to do that:
… How to Setup and Use an External SSD as your startup disk… - Apple Community
If you happen to have an iMac 27”, an internal memory upgrade is feasible without opening the iMac box. Simply adding memory won’t mask the performance of that (slow) HDD, though. There’s just too big a gap between HDD speeds and SSD speeds. Adding memory will help (somewhat), but not nearly as much as replacing the HDD with an SSD. Here is how to add memory to an iMac 27” model:
Install memory in an iMac - Apple Support
As for other potential sources of performance degradation, add-on anti-malware and add-on cleaner apps are something I’d generally recommend removing, if those are installed. Particularly if the Mac is resource-constrained, as are the iMac 8 GB HDD configurations.
If you do decide to replace the iMac 21.5” or iMac 27” with a newer iMac… The iMac 24” is far faster, and far more capable, though you will likely want to go for 16 GB memory and as much SSD storage as you expect to need over the useable life of the new iMac 24” system, as neither are upgradable.
