Same issue here. When we upgraded we were getting what is classed as 'normal' speed i.e. around 910mbps (all tested via wired network). Whilst upload speeds have remained consistent at around 110mbps, download speeds have dropped to around the 'minimum' advertised speed of circa 455mbps. Yes it's still fast but it's not what we are paying for and yet service status is showing as everything being ok.
We don't use the BT Smart Hub (thankfully) and I am guessing if I call up, that will be the first issue as I suspect I will be told I will need to connect that up before any diagnostics can be run. So, for the time being, and as the upgrade only cost us 12p more per month than we were paying, it's not a major issue but it would be interesting to know why after 3 years with BT we are not getting the advertised 'normal' speed.