Depends on how you define "much".
Depending on the specific models you are comparing, a non ((U-)LV) laptop i5 will likely be only a few percent slower than an i7 in Civ5. On other applications the difference might be much bigger, though.
For a desktop i5/7 comparison, the difference will be even less, and almost non-existent between the faster i5s and and an i7.
For modern laptops, the thermal design is at least as important as the specs of CPU and GPU.
Especially those sleek thin ultrabooks have the nasty habit of throttling massively under sustained load.
Check reviews and custumer comments before buying.