Brand new iMac(Retina 4K, 21.5-inch, 2019) running MUCH slower than my old 2013 model

Please do as den.thed instructed however my hunch is the computer is simply not suited for the kind of work you want to do. If it is less than 14 days old and was purchased directly from Apple, please return and buy one with a minimum of 16gb of RAM and the largest SSD you can afford. The base model 21.5" iMacs (8GB RAM and 5400 RPM HD) are extremely slow, the primary bottleneck is the HD. If you cannot return the computer, then your next best bet is to get an external SSD, clone the internal HD to the SSD (please use SuperDuper! or Carbon Copy Cloner) and then use the SSD as your boot drive. Then you can erase the internal HD and use it as additional storage. The RAM is not user upgradeable though.

Ah, your previous 2013 MacBook Pro had an SSD which is -much- faster than your current HDD.

To compound that issue your mac is heavily loaded with 3rd party background processes sapping your performance. Drop box file syncing, one drive file syncing, licencing keys, microsoft auto-updaters, google auto-updaters, adobe auto-updaters... all running all the time. They don't use much CPU or drive I/O, but the poor 5400rpm HDD can only just keep up with a clean install of MacOS and doesn't have much disk bandwidth to spare. (Speaking of which, Adobe Cloud usually goes in the "taxing use" category).

One option is to uninstall all non-esential 3rd party apps that come with an auto-updater and limit yourself to at most one file syncing application.

A better option would be purchasing a USB 3.1 external SSD and reinstalling MacOS onto that.

Run EtreCheckPro on that 21.5" iMac and post back here with the Report.

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

http://www.etrecheck.com/

Onedrive is Microsoft's cloud storage/file backup system. Microsoft claims Onedrive can be uninstalled by just finding it in the Applications folder and then dragging it to the trash. https://support.office.com/en-us/article/Turn-off-disable-or-uninstall-OneDrive-f32a17ce-3336-40fe-9c38-6efb09f944b0

No, once you quit a startup item it stays quit... until the next time you restart your computer.

And you are correct: the MacBook Pro's that shipped with optical drives (the last was mid-2012) all used HDD's by default. If that's the case the new iMac should have been the same speed as the old laptop (both limited by the HDD).

Theoretically the updaters shouldn't impact performance, but while I've never heard of them being a problem with an SSD they do seem correlated with slow 5400rpm HDD performance. Maybe each takes a tiny bit of performance the 5400 can't spare, maybe one of them is secretly a problem child, or maybe it's all pure coincidence.

It may be worth restarting in safe mode to see if performance improves. [https://support.apple.com/en-us/HT201262] Ordinarily safe mode is slightly slower on a healthy system, so if safe mode proves significantly faster but the next normal restart is slow again you can know a startup item is causing problems.

1st, what den.thed said. 20+ seconds to open an app isn't normal, even on the slowest baseline model, and an EtreCheck report should provide clues to the problem. Post the full report using the "Additional Text" button to avoid character limit issues when posting.

2nd, if this is the entry off-the-shelf baseline 4K model and it is within the return window (14 days from apple) then you need to return it. I've never met anybody technically savvy enough to post on technical support forums who was happy with the base 1TB 5400 rpm HDD.

3rd, as a point of general comparison which 2013 model are you comparing this to, and were either upgraded or built to order with faster drives or extra ram?

Your computer has 8GB and a rotational drive. You have 2 different file syncing programs (OneDrive & DropBox) which will constantly watch your computer's drive for new files. You also have a VPN running. The updaters and licensing key files aren't a big issue but the first three I mentioned are. Additionally, with 8GB running Mojave you don't have a huge amount of RAM left over so running Word or Excel, DropBox and OneDrive in the background, and probably Mail and/or a browser will require the computer to begin using Virtual RAM. With a rotational drive VM means seeing the beachball.

Your solution is one that I'm using right now. IT plopped an iMac without the SSD I requested and I experienced exactly what you are. Being a tech teacher, I had an SSD and an external drive handy do I cloned the iMac's internal drive to the SSD using SuperDuper! and then rebooted with the SSD. It was like sitting at a totally different computer.

My personal recommendation is a self powered external from MacSales.com (USB 3 Mercury Elite Pro) and SSDs from Crucial Tech or MacSales. You don't need a huge SSD, 256GB would be plenty for your boot drive and applications. Music and other media could be stored on the internal drive.