You will have a hard time to claim the provider has breached contract. Read the fine print and most probably you will find statements such as 'provided as is', 'network speed provided is theoretically' 'cannot be hold responsible for', and so on.
Have you already considered in contacting the service desk to discuss the problem? Sometimes the fibre optic cable is causing a concern, either on your end or the nearest hop termination end.
If you wish to terminate early bite the bullet and pay up. If you think another company provides a better service don't waste another day but you better think twice in believing that other providers are so much better.