Hi I am on 67mb faster fibre broadband and have been getting speeds around 62mb download speeds, but test today showed I was getting 29-30mb which is half what I normally get. Nothing has changed at home and I've tried everything and Talktalk said they were sending a engineer to check from box to my house. I think it may be my router. I've also noticed some lag when playing games. I have the newest router but had it over a year or close to it and I payed for it.
If you think that it is your router, although personally I doubt it, have you another router that you could try? Another possibility could be the cable between your Openreach master socket and the router. You could check that the connections at both ends are fine or, if you have one, try a different cable.
Stay safe and healthy,