It sounds like your wireless is working just fine.
It's two-way traffic, so the effective speed is generally half of the rated speed. Basically,half of the pipeline is used for each direction (computer to router and vice-versa), and that's if there's just one device on the network.
That 300 mbps is shared between all the devices on your network, so if you have several computers/tablets, etc. doing stuff on the network at the same time, each one gets a portion of that 300.
144 actually sounds about right if you just have the one computer running at the moment.