I actually tried out the thermic lance, got 100 melee, enough strength, heavy handed trait, etc. And while i do believe it does the 125 dmg it says on the pip boy, it seems to only do it over a period of 1 second. If you take out the weapon, and give a moderately quick click of the mouse button, the animation will put the lance forward, turn it on, and keep it there for about a full second. This leads me to believe it does a single attack of 125 dmg over the course the full second animation. In the end, this makes it's killing power subpar compared to other melee or unarmed weapons (main contender being the ballistic fist, which can do 104 dmg in a single quick punch, and can punch at least twice within a full second). Theres also the fact that the thermic lance has a crit multiplier of 0, meaning it is incaple of landing critical hits. Im unsure as to how the slayer perk affects the weapon (i could actually test it out in a minute), but i dont think it would have enough of an effect to make it a weapon worth using over others that are available.
A shame really, i wanted to make some melee karate master who uses a massive thermic lance like a bow