iMac speed

RAM in a 2015 21.5” iMac is soldered in place and cannot be changed by even the minor deities in Mount Cupertino, so that is off the table..

what remains is doable and cost-effective: an 2.5” SATA 6 G SSD in an external usb3 enclosure set as the boot volume. You clone, not “copy,” the entire internal drive to the external, then use System Prefs > Startup Disk to set the external SSD as boot volume.

Here are the data that show what you get for your investment. Assuming you have the base mechanical hard drive Apple chose and not a Fusion hybrid option, your current drive cannot transfer data any faster than 70-80MB/sec on its best day ever.

The external SSD option with those specs (SATA 6G 2.5” SSD; USB3 external enclosure) will do 400MB/sec. That will cause a very noticeable speed increase in actual use. No opening a computer that Apple did not want opened. And the external will serve as additional storage on your next computer.

⚠️ First check to see if you have a Fusion drive. They can go walkabout for other reasons, and the USB external SSD will be slower than a healthy Fusion drive.

the Indexed partition that apple uses for drive formats does not need to be defragged nor should it be defragged by the user. FWIW Solid State Drives (SSD) on Mac or Windows should not be defragged at all; the only thing it would do is decreases the R/W lifespan of the drive

You can install a solid state drive which would make a difference if you were looking to speed things up, the process for that model Mac is fairly involved. A less complicated option would be to add more RAM to the system, but the SSD would make the better difference when it comes to speed.

In the past I've used OWC/Mac Sales for parts like this, I dont work for them, I dont get a commission, but I've used them for years and they have provided me with great service and solid products.

https://eshop.macsales.com/shop/ssd/owc/imac-21.5-inch/2013-2019

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It simply means that your hard drive - which is the thing that does everything on your computer - has a snail like speed and there isn't much you can do about it except move everything to a fast external drive and use it instead.