iMac 27" Mid 2011 - Sometimes stuck at gray Apple screen

I'm having this bizarre issue with my iMac, which has a 2.7 GHz Core i5, 12GB of RAM (originally 4GB), a 1TB hard drive and an AMD Radeon HD 6770M graphics card with 512MB of video memory.

This machine has gotten slow over the years, so when High Sierra came out a couple weeks ago, I downloaded it and created a bootable installer on a USB flash drive. I then reformatted the hard drive as an APFS volume (it was originally HFS+) and performed a fresh install of High Sierra. The computer seemed to perform a bit faster. However...

When I tried turning on the iMac today, it seemed to start up like normal; the SuperDrive sought, the startup chime played and the gray Apple screen appeared. However, midway through the startup sequence, the progress bar slowed down drastically, and a moment later, the hard drive stopped spinning! Then, some time later, the HD started up again, then stopped. The progress bar kept filling in slowly, until it got stuck fully filled in. It was stuck like this for half an hour. This is the second time it's happened to me.

So I held down the power button for 5 seconds to turn off the computer, reset the SMC and now it started up normally! But it's still a little slow after the reformat. I think it's because of age, since this machine is nearly 6 years old.

What do you think is causing this?