As it is, the platter-based hard drive is going to make it run slowly.
That's because of the drive, not because of the iMac itself.
To get good booting and running speeds from it, you would want to add an external USB3 SSD, and set that up to be the boot drive.
Then your drive read/write speeds will jump up A LOT.
You don't have to spend a lot for the drive.
I'd suggest a Samsung t5 (remove the Samsung software and reformat it before setting it up) or perhaps a Sandisk Extreme (might have to do the same thing re the software).
500gb is "all you need".
Actually, a 250gb SSD will do the job without problems.
Put your OS, apps, and basic accounts on the SSD, but don't "clog it up" with stuff.
Leave the large libraries -- movies, music, pics -- on the internal HDD (they don't need the speed of the SSD).
But again -- if you buy an iMac with only a platter-based hard drive inside, be prepared for molasses-like performance (unless you add an SSD).
You could also open up the iMac, but it's a tricky/risky procedure.
Using USB3 will give you about 85% of the speeds you would see from an internally-installed drive, with none of the risks or hassles of prying the thing open.