People with both OS's complain of slowness. Do you currently have a ssd, this can help a lot with Lightroom. You might want to see if you can find the bottleneck in your current system before you through money at the problem.
Many people are satisfied with the performance of the Imac but for top notch performance you can get a faster PC than your Imac. If you were to build it yourself with a I7 3770K you could comfortably overclock it to 4.2 to 4.4 with out problems and gain a significant speed improvement over your Imac. If you prefer OSX and pick your parts correctly a screaming fast Hackintosh is possible. If Apple won't build the computer you need than do it yourself.