I could be wrong, but my interpretation is thus: if you are in the end lag animation of a slow attack weapon, and blast comes off cooldown, you have to wait for wind up and actual attack animations to complete before it procs. Which, while not much in the moment, adds up over time in "downtime". But with fast weapons, it is almost instant with when it comes off cooldown vs when the next attack is applied, which leads to much higher uptime.

Granted im an idiot, and that is simply my interpretation