I have only 6 mbps and have no problem with wifi and the service. That said I have a wireless N router giving me the speeds on the network. But any way you slice it 6 meg service is just that. Now may I ask how do you know that it is the wifi in your house and not the speed to the servers? The speed to the servers will be affected by many things. How many online in your area? How far are you from the ISP's switching station? If you are more that 3 miles away there you will be slower. That's just a fact. Is the equipment in your area new or old? Here my ISP just replaced the local equipment. Service has improved greatly. I can stream 2 computers and two boxes at the same time with no problems. Again at only 6 meg per second down.
My son in law has cable with 50 meg speed. He buffers all the time. Well he lives in town. I am country. He has a ton of traffic in his area. I do not. He has checked the distance and is more than 5 miles away from local switching station. I am only 2 miles away.
Finally if you are sure that it is your wifi between the router and box, then I will suggest that you check the wifi channel the router is using. Find a channel that would be better. Many signal interference problems can be solved by changing from lets say channel 11 to channel 6 on the wifi. This is just a suggestion as the more wifi is used in your area, many routers are set to a "Default" channel to send their wifi signal on. Many people do not change that and as a result many are on the same channel and creating signal problems. If I was close to my router and had less than 100%, I would start by changing channels for the wifi.
Don't know if anything I have said will help you although I hope the info does. But it is what I know about wifi. Wish you luck!