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Why is my iMac so slow and how can I fix it?

Over the last couple of months we’ve had an increasing number of people contacting us asking why their iMac is so slow. This seems to be particularly common for people with a “slimline” iMac who have upgraded to OS X 10.14 or 10.15. My answer often takes people by surprise, as typically there’s an assumption that it indicates there’s something “wrong” with their iMac. So what’s my answer?

Your iMac will be painfully slow running a modern version of MacOS if it has a mechanical hard drive.

This is often true even for  those iMacs with a Fusion drive. Even if there’s nothing else wrong with it, such as a failing drive or malware infection, performance will be awful.

You can erase the hard drive, install a completely clean version of MacOS and it will still feel like wading through treacle when you’re opening or switching between apps. It will take anywhere from 1-5mins to startup depending on how many apps you have set to open automatically.

The 21.5″ iMacs generally tend to be the slowest Macs released in the last 8 years

The “slimline” 21.5″ iMacs that don’t have a DVD drive are particularly badly effected, because they have a 2.5″ mechanical hard drive which is even slower than the 3.5″ drive that’s fitted to the 27″ iMacs. The 27″ iMacs also tend to have a Fusion drive, so the small SSD can to some extent mask the terrible performance of the mechanical hard drive.

You can’t fix a slow iMac with software tweaks – it’s a hardware limitation you fix by upgrading the hardware.

Unlike Windows, Macs generally don’t ever benefit from OS reinstalls, “clean-up” or “optimization” utilities. It’s just a waste of time and effort. All you need to do is replace the hard drive with a suitably sized SSD. Once you’ve done that you might also want to consider putting more RAM in the computer, but by far and away the greatest gain is from replacing the frankly embarrassingly slow stock hard drive.

Are iMacs doomed to always be slow computers?

Nope. They should typically be faster than equivalently priced MacBooks, assuming that both machines have a SSD. They normally will have faster processors and video cards, plus they don’t thermally throttle as quickly as MacBooks, meaning they can work harder for longer.

The problem is that people who use desktop computers often use it as the “family” computer, tend to generate more data and hence Apple prioritised fitting a large hard drive over a fast hard drive. Because Apple significantly over-charge for their SSDs, many customers  looked at price difference, didn’t understand how badly it was going to affect the performance and their enjoyment of the computer, and crippled their lovely iMac at birth 🙁

iMac hard drive replacement service

So if your computer is so slow it’s driving you crazy, you have a simple choice. Buy a new Mac that has a SSD with sufficient capacity for your current and future needs, or upgrade your Mac with a suitably sized SSD. Over the years we’ve done literally hundreds of SSD replacements and in all that time we’ve never had anyone who’s not been shocked by how much faster their computer feels.

Typically you’ll be looking at a 10-15% of the cost of a new computer. Not cheap admittedly, but massively cheaper and I would argue more convenient than buying a new computer.

Sick of watching the “spinning beach-ball of death” every time you try to do something on your iMac? Don’t want to have to buy a new Mac? Live in Bath, Trowbridge, Melksham, Devizes, Frome, Westbury or the surrounding villages? Give us a call on 01380 830224 and we’ll give you a quote that will transform your slow iMac.


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