Lag over xbox live

Since I switched to Sky unlimited broadband most online games on xbox live have become unplayble due to lag. Street Fighter for instance runs really slow with huge input delay. This has happened in 2 different addresses as I brought the router / contract with me. There is no issue of anything else in the house using the bandwidth and the lag happens no matter what time of day I use it.

I use a wired connection between xbox and Sky SR101. I previously had BT broadband and that was perfect.

Is there any reason to assume that upgrading to sky fibre would provide a lagless connection?

Thanks in advance for any help.