This thread actually answers the question pretty well:

http://www.bikeforums.net/touring/52...ng-frames.html

in particular,

"Different styles of riding require different positions... if you're a triathlete as opposed to a pure cyclist, you will ideally want a position that takes advantage of your strongest muscle groups (e.g. hamstrings iirc). As a result, TT frames have very steep seat tube angles (e.g. 78º).

Touring bikes have slacker angles primarily for positional comfort. Also, the more upright position gives you a better view, albeit at the expense of worse aerodynamics. The design also adds a little bit of flex (and therefore comfort), since the seat tube will be longer. IIRC slacker angles also uses the quads more, which is better for seated climbing."

Because the mtb I'm using as a touring bike has a very slack seat tube, and a very long wheelbase it is both comfortable and slower, because I'm using primarily my quadriceps. So I'm powering up hills very efficiently but when on the flats I'm a lot slower even if I'm exerting myself the same.

So the answer to my original question is pretty much just that.

For those interested, and as this is the bike fitting forum I think this is relevant, this article about frame geometry is of some interest as well:

http://www.cyclingabout.com/understanding-bicycle-frame-geometry/

thanks for all the advice, appreciated!