Slow startup MacOS High Sierra 10.13.6 on iMac Mid 2010

Until recently I had the same model--Mid 2010 27-inch quad-core i7 2.93ghz. For most stuff it remained fast until the day it breathed its last about six week ago.. However, boot and load times were long. It's not the processor, it is the speed rating of the SATA connection between the logic board and the drive.

2010 was the last year that Apple used a 3GBps SATA connection in iMacs. The next year they changed to to a 6GBps SATA rating. You can't get to the 6GBps speed on yours but you can probably find a private Apple-trained technician to install a 3GBps SSD like the one here (a kit with all the needed tools and extras):

https://eshop.macsales.com/item/OWC/KITIM09HE1TB/

and see a decent speed boost.

⚠️ However, before spending money on hardware upgrades we need to see if you have any known software issues. You can speed a lot of money on hardware upgrades, but if you don't consider software issues first, that money buys you little.

Please post the Etrecheck report you have. It was written expressly for being posted in these forums by a long serving a trusted contributor here, and will not reveal any personal or secure information to others.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find) and, when its report displays, click the "Share Report" icon and then "Copy " from the resulting dropdown. Do not highlight the text in the report before copying. That will garble the formatting making the report hard to read and evaluate.

NOTE: The recent changes to the forum software requires you use the "insert text" icon to embed the report into a post:

Thank you for posting the report. I'm pleased to say I see a nice, clean Mac! Now we've eliminated software issues.

But why was this iMac booting so fast with Mountain Lion or El Capitan?

High Sierra has more content than did earlier OS versions. My 2010 came with 10.6 and I left it there a long time for compatibility reasons. I jumped straight to OS 10.9 Mavericks when the compatibility thing went away but did not notice a bit of slowdown until I got to 10.12. It wasn't bad but was definitely there. It did not effect execution, only boot and load times.

In full disclosure, I admit that when I upgraded my 2012 MacBook Pro to a 6GBsp SSD, everything else in the house seemed sluggish. The MBP was the entry-level mode that had a 3GBps mechanical drive connected to the computer's 6GBps drive bus. With a dual-core 2.5ghz i5 it was definitely the "also-ran" in the Mac horserace at our house. Entry-level equals "keep the price point down with cheaper parts," I suppose.

After I upgraded it to 10.12 and 10.13 the slower drive was really apparent. After the SSD in late 2017 the tables turned; the low-cost MBP felt like the fastest. It is the main reason that when I replaced my 2010 iMac I got one with a 1TB SSD. (27-inch 2017 5K 4.2ghz quad-core i7).

The install video for the SSD kit I listed shows the job as "advanced" for the home installer. If you want to evaluate your comfort level with the upgrade, the video is here.