Q: would 1.4 Ghz be way slower than 2.7 Ghz?

Im thinking of buying a iMac so i can stop using my mothers Macbook pro,  its reletivaly faster than my HP so I use it instead ,anyway, the macbook has 2.5 Ghz at a satisfying speed or whatever. The cheapest iMac is 1.4 Ghz, I'm not the richest person... is there a huge difference in speed for example casual gaming? I would use it for gaming, and some streaming of youtube.