I was on a 16Mbps plan when I was on adsl2+ and i would ping 23 ms to a local server but then I upgraded my plan to 50mbps and bought a vdsl2 modem router ( tp link td w9970) and ever since I always ping 30 ms to the same server. I know speed does not affect ping but I dont understand why my ping increased. I live in Egypt so my Isp sucks(there is only one isp anyways bcz of the government so I cant switch isps) and they tried dropping the connection from their end and bringing it back but nothing worked . I also am sure this is not a wireless problem with the router as I test using a lan cable.
You may be marginal Signal to Noise Ratio on getting the higher speed. The connection then has to be switched from Fast Path to Interleaved to give you a working signal. You can try dumping the information out of your modem to see if that is the case.
You will then have to work with your ISP to see if they can put you back on fast path even if it means lowering your speed slightly to keep the SNR margins.