Wifi connectivity issues

I have often wifi connectivity issues. My mac is connected to a specific wifi network but have no internet or the internet is extremely slow. I connect to the same network from my phone at the same time and internet works just fine with very good speed. It has happened with different wifi networks. Some times for some period it might work fine but next minute it goes off again. I have tried restarting, forgetting the network, renew DHCP and reset the NVRAM/PRAM and SMC. Nothing worked.