What can be replaced/upgraded in a late-2009 iMac to make it run like a 2014-2016 iMac? I have a late 2009 iMac. The specs are in pic below. This machine got a new hard drive last year and is running 12GB of memory, yet it often lags and is not as zippy as I would like.

I am curious if there is anything that can be replaced or upgraded to make it perform like a newer machine or are the un-upgradable parts just too long in the tooth? Thanks for the advice.
One the trade-offs to all-in-ones is not being able to upgrade most of the parts. You could try upgrading to an SSD but you're not going to see huge performance gains. The primary bottleneck at that point will continue to be the CPU (and possibly graphics).