Back on Thomek post: I had the same (experience) when fighting against a flamburger. I have a balanced build and I use heavier tool than katana (2H swords, 2h axes) and it happened to me that the flamburger could outspam my attack. I did not expect such weapons to be fast.
I also had the same experience fighting against a glaive wielder (Templar_Tommyyy to not mention him).
The stun effect is usually obvious and I did not feel that my character reacted slower (but I cannot tell for sure).
Actually, I think that the flamburger (like good polearms wielder) has a perfect mastery of turning into the swing. The arc of danger means the flamburger can hit very early in his swing, so to compensate his low speed, the flamburger turn the body when attacking to hit at the start of the swing. If performed with good timing, the flamberge can hit first (to the extent the opponent does not turn his swing as well). This is the art of the flamberge.