The problem with comparing XLN's speed and DFBB's speed is that you have different people weighing their opinions.
LHC is a special case when it comes to viewing techniques. If the technique is covered in his DG9J training, then he can see things very clearly, even at incredibly high speed. This is apparent in his fights against Yue Bu Qun and DFBB. But on the other hand, he couldn't see clearly in Ren Wo Xing against the Shaolin Abbot, yet Xiang Wen Tian had no difficulty observing the fight. Now this is kind of inconsistent, as DFBB used much more inner power on his needle than either Ren Wo Xing or Shaolin abbot is capable of. But since he used a "weapon", LHC apparently can see things well. So we'll take it at that.
Basically, LHC has observation skills *beyond* his natural ability, both in terms of pointing out weaknesses and speed. Whereas most other fighters' observation skills are consistent with their fighting ability. So even though LHC can see DFBB, but many top fighters in ROCH couldn't see XLN, doesn't mean XLN is faster. It just means that LHC has extrodinary observation skills.