Certainly your old computer is a problem and there may or may not be upgrades that can speed it up. More memory is one way to speed things up, but there is no guarantee that the problem is memory. A faster CPU will certainly help, and I would recommend that before increasing memory, if your computer can handle it.
You don't say what aspects of Lightroom are slow. That makes a difference in the path forward.
But honestly, I think you are up against an insurmountable task, I don't think a 2009 iMac will ever give you reasonable performance with newer versions of ightroom.
I am running Adobe Photographers Package PS CC 2015 and LR CC 2015.1.1 and my system is
OS X Yosemite 10.10.5
MacBook Pro (13" Mid 2009)
Processor 2.26 GHz Intel Core 2 Duo
Memory 8 GB 1067 MHz DDR3
Startup Disk 1TB
Graphics NVIDIA GeForce 9400M 256 MB (GPU turned off)
Works fine for me, its not a speed king, but performance is relative to the hardware specs available.
That core 7 should be quite usable with the current Lightroom and Yosemite. My guess is that your memory is just way to small. I would at least put 16 GB in that. Its hard drive is probably also very slow but memory is probably the main limiting factor.
Oh and also don't forget to turn off GPU acceleration in Preferences->performance. This actually will slow down Lightroom on all but the very newest and tastest graphics cards. A 2009 iMac will become glacially slow with it turned on if you do anything at all with brushes, graduated filters etc. in Lightroom.