I made a mistake in my assessment of the 133 diversion, in thinking it terminated at Smithfield: was confusing it in my veteran brain with the 59 end point. Nevertheless, the 133 runs less frequently than the 521 during the peaks, arrives at London Bridge northbound with plenty on board already and there's quite an extra walk to reach it too from London Bridge station platforms.
With this newly rediscovered love of branding on the TfL bus side, it's pertinent to mention that the 521, and 507, were part of the most successful rebrand in post-WW2 London bus history to date, the Red Arrow network, created and executed by people within the Bus Operating Dept with no input sought or obtained from advertising agencies/ image consultants etc. Why does it no longer exist? I suggest its disappearance coincided with B.Johnson's election as Mayor, because the 507 and 521 introduced the 'bendybus' to Central London's streets and we all know what he felt about them despite probably never travelling on one. Those routes were the first to lose them too.
Regarding the speed of buses, the traffic schemes at Aldgate, Elephant and Castle, Bank junction etc and the length of time in their execution showed City Hall's contempt for bus passengers.