I doubt it is the HDD. I have a 2012 i5 with a 5400 RPM drive and it's very snappy. My common programs (Photoshop, iTunes, Mail to name familiar ones, as well as many third party) launch in one bounce animation, basically a second or two. The Yosemite progress bar begins immediately on start-up and it takes about 25 seconds to arrive at the desktop with everything ready to go, and I have 3 start-up applications. It shuts down almost instantaneously, and restarts with a combination of the two.
What I will say, and I hope this helps, is that when I had the stock 4GB of RAM installed in it, it wasn't "slow" but it was at least noticeably less quick in terms of operation before I added an aftermarket 8GB kit. I realize that the 2014 models have been neutered in that area, so perhaps at least an 8GB BTO configuration will help you out if you are still interested in the product line after your return.
I realize that the HDD is more than likely always the prime suspect on everyone's mind, and not saying a SSD upgrade won't speed the experience up because it will, but this has been my experience with the same device, only slightly different model, and I hope it helps you.
I think it is extremely ignorant to claim that the reason a computer doesn't run something that it is optimized for is because it doesn't have a SSD. The population seems quite spoiled by these becoming mainstream. I have used them and I still find mechanical drives to be more than viable, even though my clear opinion is that SSDs are superior in more than one aspect.