I don't think that building with SysX is slower than with steck.
I don't know how you are building, but once the connectors are plugged in, the building is exactly like with steck.
Steck is a cool system, without doubt, but it has some flaws also. Especially, I dislike the sockets that hold the floors and starwairs. They have a tendency to break. There I love SysX because almost all parts that can break can very easily be replaced. I have a lot of broken steck floors at home, but never ever a broken piece of SysX.
If Playmobil would decide top reintroduce Steck I would be happy. But I would like to have all these nice features from SysX. I just feel that the way of SysX replacing Steck is the more realistic one (in terms of Playmobil basically discontinued steck almost 10 years ago now!), and I do like the new SysX-castles (esp. the rock castle and except of the huge junk pieces). If they would keep it more modular, SysX would be just as flexible as steck was.
Several people are complaining about the walls falling apart with the Kings Castle. I do not have this one, but clearly this is a flaw that can be improved. Either by an additional connector slot or simply by introducing a more rigid connector. They have something like this already!
The owners of the Kings Castle should try these connectors:
30 26 1780 - System X plugI would bet, the walls hold together more strongly.
best,
socrates