Macbook Pro Retina 13" 2015, very slow, freezing, and WIFI keeps coming off.

Hello I have two problems with this MacBook I got from a friend. First, my laptop is extremely slow, like I bought this as an upgrade from the Macbook Air 2015, but it has been anything but that. The computer freezes constantly, I can't watch Youtube properly as the video just buffers endlessly, and the WiFi isn't the problem as the sound is always "ahead" of the video, and the video pauses until the graphics catches up with the sound.

I checked the Activity Monitor, and when everything is closed, the memory used stays at a constant 4 GB. CPU is pretty low at 15%. In addition, I checked Disk Utility to see if I'm using all of the storage, but it looks like there are two OSX installations, I am not sure though.

I downloaded an "iCleanMemory" application, and it seems like when I "optimize Memory" it's max free memory is 3.4k, but goes down in increments of 4 MB per second.

The second problem is, the WiFi is somewhat weak. All other devices in the house connect to my house Wifi just fine, but for some reason, every 14 minutes and ~20 seconds exactly, my computer's adapter times out, and I need to reset it. I am not sure why.