iMac lag

Hi Kamal!

Excellent, you have plenty of storage. Try booting into Safe mode (HT201262) and then running https://www.malwarebytes.com - the free version only. Then look for the Malwarebytes-3.dmg file in your Downloads folder, open and install. Once installed, you open to run the scan. Malwarebytes does offer paid services as well but I find that the free version works perfectly fine.

You can also try testing out in a new administrator user account. Apple menu > System Preferences > Users and Groups > click padlock to unlock > "+" add > Administrator account. You can name the test account anything as it will be just for testing purposes but you just want to make sure it has administrator rights.

Once test admin account is added, go to Apple menu > Log out and log into the new account. You will want to test to see how it runs. If everything seems OK in the test user, this means that something is hiccuping with your main account. I would then reset your Home folder permissions as shown here: https://support.apple.com/en-us/HT203538

Also, I would make sure to clear out any login items (automatically open when booting up) as this can cause some serious lag. Apple menu > System Preferences > Users and Groups > Login items tab > highlight each that are showing and click on "-" to remove

Hello! Whatever you do, please don’t download Clean My Mac as that will make your Mac even more slow. I agree with removing any antivirus software as your macOS protects you.

I would try the following:

1. check login items and remove in settings > users and groups > click your name > login items > remove all
2. boot into safe mode to test: https://support.apple.com/en-us/HT201262
3. run malwarebytes ( free version). This is the only malware scan Apple will recommend when contacting them: https://www.malwarebytes.com/
4. check available storage: 🍎 menu > about this Mac > storage

First thing remove Norton antivirus app from your iMac, it is completely useless with all MacOS. It will conflict with MacOS features, making things worst. Then download and run Etrecheck on your iMac, then post results in this forums. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

the reason why it takes so long is because all the hardware inside is very old. With the TB hard drive it will take a long time to start up and the Intel Core 2 Duo processor is very weak. I you want to try and speed up the Mac you could try downloading clean my Mac X. This will clear the unwanted files, you can free up Ram and do maintenance on it. You can also seeing how much CPU you're using.

you could try downloading clean my Mac X

Under no circumstances add that load of meretricious muck to your Mac. It's dangerous, and risks significant dataloss. Avoid at all costs.

It will not clear "unwanted files" - you can do that yourself, "free up Ram" is gibberish, the Mac does that anyway, and no, your Mac does not need maintenance. You can see how much cpu and memory you're using and a whole lot more, for free, with Activity Monitor, in your Applications/Utilities folder.

Seriously, search the forum for commentary in that app.

It takes too much time too boot up

How long is 'too much time"... an old Mac like that could have a failing hard disk.