IMO this question is either "Does Madame Tussauds London publish waiting times" or too broad. It seems they do not publish (though OP may wish to call +44 (0) 871 282 9200 to check whether such statistics are collected even if not shown on their website).
Other evidence (in line with @Gayot Fow's Comment) is that delays are variable, for example as mentioned on Trip Advisor.
In view of "Two for One" tickets at least the time of day makes a big difference. Even pre-booking may not help:
.went at 10 a.m. on a weekday............went right in.........the only lineup was for people who had prebooked tickets
though admittedly from four years ago.
Factors that I would expect to make a difference include train strikes, school holidays, weather, exchange rates ... and almost anything else imaginable.
Is this true
how do the queues compare to the "normal" walk-up queues?
It depends. They may be shorter or longer, even perhaps about the same.