Having considered using bus from Waterloo to Euston the other day, at around 30 minutes, it was just too long a journey to contemplate, with no guarantee of reliability.
Indeed, the TfL journey planner suggests that the KGX-Waterloo bus journey will take 34-36 minutes, over a wide range of times across the day. It does not seem to make any particular allowance for prevailing traffic conditions. Meanwhile the via Oxford Circus underground journey takes 10 minutes (possibly plus station access time).