There is one obvious reason your older machine could be slower and that is it only has 4GB of memory. Open Activity Monitor on that machine, click on the Memory tab and look at the bottom of the screen. The key information is Memory Pressure (a graph) and Swap Used. If memory pressure is high and especially if it is any color but green you are pow or out of memory and are using your hard drive as a substitute for more memory. That could really slow the machine down.
