Got this bug today too. Weapon (legendary katana, precisely) became more and more slowed, after some use (slicing and dicing some gonks). To zero attack speed and zero dps. Mantis blades too. Occured just today, after almost 60 hours in game. Maybe some newly picked talents is the cause of this problem? I just picked talent in cold blood, which remove cold blood stacks not all at once, but one by one.
Fix is the same, as for users above - just reload and all is fine (but it's tedious and frustrating).
EDIT: tested via tabula e-rasa respec - problem, at least for me, is in perk COAGULANT in cold blood tree- when i removed it, all became fine again. Melee weapons and cybernetics stopped losing attack speed after each fight.