I replaced the spinning disk on my 28" iMac 5K Retina with an SSD. However, not wanting to risk dismantling the machine, the SSD is simply in a USB3 enclosure velcro’ed to the back. It’s now brilliantly fast, and it was brilliantly simple to do.
A friend of mine wants to do the same, but his Mac is rather older than mine, with only USB2, rather than USB3, to go with the Thunderbolt interface. As Thunderbolt enclosures seem to be a bit like hens’ teeth his only option if he is not to dismantle the machine seems to be to connect the SSD via USB2 - but the concern is whether a USB2 connection will be fast enough for the SSD to offer any significant speed improvement over his (SATA!) spinning disk. Has anyone here ever tried that?