Fan running on my iMac mid 2011

Has the original hard drive been changed or serviced? There is a temperature sensor attached to it that, if removed or disconnected, will cause the fans to run up to full speed. This is a very common oversight when techs service or upgrade 2010 and 2011 iMacs.

Also it seems to have slowed down its response to commands.

Slow comes in many colors. The most common complaint is that the computer is slow to start and apps take a long time to launch but, once launched, apps run at normal speed. If so, that is the hard drive.The mechanical drives used in 2011 21.5-inch iMacs were, although faster than the ones in 2012-2019 iMac 21.5-inchers, still mechanical and slow compared to modern Macs with solid state drives. We can see the data needed to confirm this diagnosis if you post an Etrecheck report.It is available available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support. We can see hard data about drive performance, software issues, 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):

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: Changes in late 2018 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 “Additional Text” window:

Try resetting the SMC How to reset the SMC of your Mac - Apple Support

A simple solution as an experiment, just use a hair dryer, but without heat, and direct the dryer to a fan, as the accumulated dust may be the cause

Not so simple for that iMac. The hard drive fan in that model is extremely difficult to access, requiring removal of almost al the internal parts including the logic board. This is a difficult task for an experienced tech and nearly impossible for a novice end user.

A popular self-service site shows 46 steps to reach the HD cooling fan, rating it "difficult."

We do not have this model of Apple computers, but I gave advice to traditional computers, and it is often successful advice. As for you, I am sure you have more experience than me, and your words are correct. Thank you, Alan.

You are most welcome. Third-party service sites are useful for seeing the accessibility of different model for user serivicing. We happen to have that same iMac model in service but none of us can be expected to have access to all models.

This is why you and I work together here.

Oh, and welcome to the forums!

A simple solution as an experiment, just use a hair dryer, but without heat, and direct the dryer to a fan, as the accumulated dust may be the cause