2017 iMac 21.5 been slow since I took it out the box. 50-80 read write speeds. how can I fix?

The problem is likely the 5400 rpm HDD.  The drive is the bottleneck.  This issue is common and pops up every day.  The fix will be to use an external SSD as the startup drive. You'll see a noticeable improvement in performance.

I and a lot of the Community veterans will recommend OWC/Macsales as a great place to buy Mac peripherals and upgrades.  (unsolicited)  Please see > Mac Drives, Internal, External - macsales.com 

If you'd like to give your Mac a little more life, couple an ext SSD startup drive with MacOS Monterey and I think you'll be surprised how well your Mac continues to run.  I can usually coax my Macs to ten or eleven years without breaking a sweat, and I've been doing this a long time.

Or, buy a new Mac!  It's always fun to get a new toy. 😃

Buy Mac - Apple 

Don’t neglect making a Time Machine backup before you do anything else.  If something goes sideways, and it may, it’ll save your butt!

Back up your Mac with Time Machine - Apple Support 

Also, to verify that the drive is the problem and not something else – that we may be able to help remedy – I recommend that you run an EtreCheck evaluation of your Mac. EtreCheck is a safe and recommended utility from a trusted developer and respected ASC contributor. The diagnostic app gathers important info about your hardware and software and reports it back to you. It is very highly regarded and meant to be used with Apple Support Communities to help others help you troubleshoot and resolve your Mac problems. The EtreCheck report will not include any personally identifiable info.

Please navigate to EtreCheck.com and download the free version. Be sure to enable full disk access when you install the app. Once you’ve run the app and created your report please post it with your reply to this message. 

You must upload the full report. To see how, please click >  How to use Add Text when posting… EtreCheck Report.

We can use the EtreCheck report to look for the things that may be causing your problem and advise how to correct them.

The entry level 21.5" iMac's have a really slow 5400rpm HDD and that is as fast as they go.

The best way to improve the performance, is to startup from an external SSD.

To do that, see > Use an external SSD as your startup disk … - Apple Community

Download and run the EtreCheck utility that I linked above. That app will evaluate the hardware and the software that is running on your Mac and provide a report. The report will contain no personally identifiable info, but it will provide a host of info that can help us diagnose your issue.

To determine if you have an SSD installed, you can Option-click the Apple logo () in the menu bar, then select System Information. On that screen, select Storage under the Hardware section and see the Medium Type info.

On my2019 iMac I've installed the OSS to a fast external SSD...

OWC Envoy Pro FX - The Fastest, Most Compatible Drive Ever Made (macsales.com)

Good luck my friend! :)