As far as cleammymac stuff, stay away from MacKeeper first and foremost.

The Mac OS takes care of itself pretty well, but sometimes you need something like OnyX to perform some cleanup. As fas as the computer problem, could be a number of things. Using OnyX may help. If that doesn't clear things up give more detail about what version of the OS, memory, available HD space, what you use the computer for, what apps you use often, and which ones you have problems with.

For the wireless question, if the iMac can see other networks but not the MiFi, can you see the MiFi on your phone's other device's wireless network lists though? If it can be seen on the phone/device but not the iMac it's something with the iMac. If it takes a while to show on both, it's the 4g unit possibly. May be going bad, or a setting needs to be changed on the MiFi.

Go to on the iMac and see what your upload and download speeds are. They have an app for mobile phones, check it there too to compare with the iMac.