Tires, as everybody else suggested.
I am not familiar with the bike you have, I do not know if it is 700c or 26 inch. If 26 inch, maybe look for some fairly supple 40mm wide tires, something like the Vittoria Randonea Pro. If 700c, there are a huge number of good tires, so I won't bother suggesting a brand or model. Width, could pretty much be up to you and you might consider for loaded touring a wider set and for around home riding with minimal load a narrower set of tires.
The thing you are looking for is a tire that is very supple, the tire carcass and fabric will feel very flexible. The bead does not matter, so a wire bead will not be slower than a folding kevlar bead. The key is the sidewall and the tread, you want a tire that takes very little energy to deform it. After all at 15km, you are deforming about 9 meters of tire for every second that you are riding it, so you want a tire that will not consume energy to deform it.
I find that my vintage Columbus tubing fast bike with skinny tires feels much faster than my Thorn Nomad (heavier bike than yours), but when I get home after a bike ride on flat and level terrain, the actual time spent on the road was not much different. Although my fast bike is not in the ultra light carbon class of bikes, it probably is almost as fast as those bikes are on the flat and level. Note that I said flat and level, you did not say if you are doing big hills or not. It is the hills where the weight of a bike becomes much more noticeable. If you are frequently wearing out brake pads, then maybe you should consider a lighter bike.