Speed question

In general usage, a lot. Because the rMBP has an SSD.

iMovie, quite a noticeable increase, because of Intel QuickSync in Haswell.

 

The first scores 6356 on geekbench, the second scores 8626, a roughly 35% increase in raw processing power.

You'd see that difference in CPU heavy tasks such as video and photo rendering, finite element analysis and working with very large datasets.

 

If you just want a simple speed upgrade, I would install an SSD in your iMac. Guarantee the difference will be more than satisfactory.