Why is it that my 2014 MacBook pro faster than my 2017 iMac
Pretty much any SSD is going to be much faster than an HDD.
You're looking at much faster read speeds on an SSD.
A typical 7200 RPM HDD will deliver a read/write speed of 80-160MB/s. On the other hand, a typical SSD will deliver read/write speed of between 200 MB/s to 550 MB/s.
Source
But, fret not! You can add an SSD to your iMac.
Here's a link from the Apple's website. Here. I'm sure you can find a DIY guide there somewhere. Or check with your local Apple Authorized Service Provider.
But answering your question, if your 2014 has an SSD and your 2017 has a HDD, the one with the SSD is going to be faster, regardless of the year.