Can I use a smaller external monitor to improve gaming performance on my iMac?

Hello, I have a 21.5" late 2013 iMac with the intel i5 2.9GHz processer and NVIDIA GeForce GT 750m GPU. When I purchased it I assumed the i5 and GT 750m would pack enough power to run most of the games I play and long story short I was wrong (surprise). I like the iMac a lot but with new game titles coming I'm constantly wishing I did more research and built a true gaming PC because this luxury computer isn't cutting it. I mostly want to run World of Warcraft which is far from the most demanding title but all this Mac can handle is "fair" settings on full 1920x1080 resolution. BUT, if I turn the resolution down to 1600x900 I can run high settings with 65fps, which is awesome, however the smaller window drives me insane. So my question is can I buy a smaller resolution monitor that I can connect to the iMac and run my games on? I really don't feel like spending $400 and building a dedicated gaming PC when I shelled out $1500 already for my mac, and maybe run WoW on high settings. Also, if you can recommend a smaller monitor that would be great. Thanks! 🙂