Hi there,
after challenging and pushing my new X1 I mostly come to the conclusion that the volumetric flow of the filament I’m using is part of the limiting factor to gain more speed. Depending on the model (I mainly design functional parts which are easier to print) I don’t gain a huge amount of time compared to my fine-tuned MK3. This does not mean that I’m disappointed by the X1 Carbon - but I had hoped for a bit more speed increase.
So after some testing I see that the filament seems one of more limiting factor. Currently I have very good experience and results with Polyterra PLA (with a max VF setting of 25). But I would like to know if there are other brands that I may can push the limits a bit further? Anyone has experience with some HF and if they are available in Europe? I don’t mind testing 
Thx