Slow Mac

The best practice is to look for software issues first when a computer is slow. If you have some useless utilities installed that slow a Mac, your computer will still be slow after spending money on RAM or new drives. The Kanex adapter costs US$80 on Amazon, more than some external hard drives.

https://www.amazon.com/Kanex-Thunderbolt-eSATA-plus-Adapter/dp/B00LOLBBQQ/ref=sr_1_3?keywords=kanex+ktu10&qid=1558795580&s=gateway&sr=8-3

It looks to me like a great option IF you don't have software causing slowdowns.

Yes, some 2011 iMacs can be slow--we have a slow one too--due to a slow internal drive but assuming that first is skipping a step.

You've not told us much about your iMac other than the year and OS version. The screen size makes a difference in what hard hard drives are installed and that affects performance. 27-inch models usually come with faster drives than 21.5-inch models.

So where to start? We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck, available free from the Mac App Store here:

https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12

We can also see hard data about drive performance and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

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 from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.

Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

NOTE: Recent changes to the forum software require you use the "additional text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Add Text” window:

Also see this excellent user tip on posting text reports like EtreCheck. It is better illustrated than mine.

How to use the Add Text Feature When Post… - Apple Community

use an external thunderbolt drive which has an SSD drive. A Lacie rugged will do it fine. You can swap out a mechanical drive for an SSD drive. You should take your 2011 to 16 gig of memory. You can purchase a thunderbolt to USB3 external to rid yourself of USB2. Called a Kanex and model #KTU10. Then use your internal mechanical drive just for storage of data for back up.