Before you can be released penalty free from a contract, attempts to get your line above the minimum threshold are made, so if there is anything ‘impacting’ the lines performance, that would be attended to during this process , so it’s a moot point anyway.....FWIW, if you had poor internal wiring , or still had the bell wires connected on extension sockets , or had your router connected to a cheap telephone extension cable , that is likely to ‘impact’ your lines performance , so your speed may well fall into the impacted range, if your router is connected in the master socket test port, the line isn’t likely to be impacted, and you should expect clean line speed, so any initial complaint about ‘speed’ , would result in the ISP getting you to connect to the master socket test port....obviously if speed jumps to above any minimum, whatever the issue is, is yours and not with the ISP or OR
