Recumbent trikes can be slower up hill because, well, with a nice comfy seat, and low gears - why not?
On steep hills, a common technique on diamond frame bikes is to stand on the pedals, using arm and upper body strength to provide short bursts of power. Here the legs are doing some of the work, but by being relatively straight, they are allowing upper body strength to be used. This is not necessarily very efficient - but it can get you up short sharp hills quicker. On a recumbent, while it is possible to get a lot of power to the pedals by pushing against the back of the seat (like doing a bench-press at the gym) - all this work is being done by the legs, with no help from the arms/upperbody.
In other words, I think that going up hill on a recumbent is like going uphill in the saddle on a diamond frame bike. On both types of bike it is possible to develop a smooth pedaling style that is more efficient. The difference is that on recumbents you've no other option, and on diamond frame bikes you don't have to - you stand up on the pedals instead (which is what most people do).
Does that make any sense?