A lot of things can cause the system to hang, doubtful it's the cpu. The i3 is much better. Did you reuse the hard drive from the win7 machine and if so did you format and do a clean install of win8 or did you upgrade to win8 over the top of win7? Have you updated win8.1 with all the updates? How much ram do you have in your new system and what type of hard drive are you using (ssd, hdd - if hdd, is it 7200rpm, 5400rpm)? A full list of system components would help if you can post your system build.
Have you checked your task manager to see what all else is running, how much ram % is used, what your cpu usage % is? If cpu is at 100% because of gaming and other things running it can cause hiccups, if your ram is close to or 100% used it can force your system to use the swap file on the hard drive which is much slower and can make an otherwise fast pc feel like it's crawling in mud.