Depends what you do. I went from a 2014 i7 1TB SSD iMac to a 2020 i7 2TB SSD iMac, and don't feel it is much faster for day-to-day work loads (which is 95% of the time). It is probably 50% faster for CPU intensive workloads.I need some advice. I have a 2017 Fusion 27" iMac. I am wanting to upgrade but I don't want to give up the screen size, and I also don't want anything bigger than a 27" if thats what they announce in the future. I am considering getting a 2020 iMac to get more future proofing than what I currently have. How much faster will it be than my current iMac?
But note that both these machines had internal SSDs. I suspect the Fusion drive is likely the biggest speed bottleneck in your machine. But this can be (partially) addressed by getting an external SSD. A lot cheaper than a whole new computer.
Best way to find out is to take advantage of the 14-day return period.
I really like my 2020 iMac that I bought a year ago, but I would hesitate to do so today, with its replacement rumored to be soon. I also expect there will be lots of used/refurb 2020 iMacs available.