On Friday I was on the 390 from Victoria to Euston, a journey that usually takes just 20 mins, was nearer to 40 mins, there was little traffic and the lights were in favour of road vehicles, the only bit of traffic was toward the end of my journey on the Euston Road, but that’s always heavy in traffic anyway and wouldn’t have added any more than 5 mins usually, I agree that buses are moving much slower.
Because congestion has gotten worse. Obviously buses aren't a segregated transport. The road network in London really can't expand (nor should it anyway in my opinion and active transport modes should take priority) so if you keep throwing in personal cars - the average occupancy of which must be <2.0 people - congestion increases exponentially, not in linear fashion due to the impacts upon surrounding junctions & roads.