apps take too long to load on catalina 10.15.5

Technically there is nothing wrong with the machine, what is wrong though is you got the wrong machine for the type of apps you have installed on it. You bought a base model 21.5" iMac which is only intended for surfing the Internet and e-mail, not running apps like you have installed on it. To be honest, I don't think it should be sold at all. It has 2 bottlenecks, the first is it only has 8GB of unupgradeable RAM which is considered the bare minimum to run Mac OS. The second and most major issue is it has a 5400 RPM HD which is glacially slow. It's like buying a Corvette and putting 500cc 4 cylinder engine it with 100 hp. Its simply not going to run like a Corvette with a 550HP V8!

The only thing you can do with your current computer to improve performance is to buy an external SSD clone the internal HD to the SSD and then use the SSD as your boot drive and primary drive and use the internal 5400 RPM HD for additional storage. Its either that or live with what you have or sell it and buy a properly configured iMac for the type of work you want to do. If you are interested in that route and want to stay with a 21.5" iMac, get one with a minimum of 16GB of RAM (more is better) and the largest capacity SSD you can afford.

I second rkaufmann87's recommendations. You also have some unnecessary software installed: WebRoot. There is no reason to ever run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac.  This user tip describes what you need to know and do to protect your Mac: Effective defenses against malware and other threats - Apple Community.  Uninstall it according to the developer's instructions.

Dropbox and OneDrive can be performance inhibitors. Remove them from launching at startup, i.e. the User Login Items preference pane, and run only when needed then quit them.  

To add to rkaufmann87's recommendation: if you get a Thunderbolt external SSD that would be very, very fast. However, it's much more expensive than a USB 3 SSD which is no slouch.

You can download and use Find Any File to search for any files with the application's name in the file name.  For example for WebRoot software you'd do the following search: 

1 - Name contains webroot

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.

If you find you can't drag the files to the Trash bin because the app is open boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and run Find Any File from there.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

Post an EtreCheck report of your system and we can look for obvious issues.

I would go with the following:

https://eshop.macsales.com/item/OWC/ME6CM6EPT01/

To clone your internal HD to the SSD use either Carbon Copy Cloner or SuperDuper!, you can use the free version of SuperDuper! to clone to the SSD, then start from the SSD and set it as your startup drive and you are in business.

After the next backup open the TM drive and look inside to see what drives are being backed up. TM should backup the internal drive and all mounted external drives as this screenshot shows:

Thanks for the follow-up. You can see which drive Time Machine is using by doing System Preferences > Time Machine. In this example, my external OWC drive named "Time Machine 2T" is set for all TM backups:

"Select Disk..." lets you specify which external drive to use.

I understand.