Glad to hear your enjoying it so far. The first boss once he reaches a third stack of adrenaline you have to avoid him until he starts rushing you then you run him into one of the broken pillars on the ground to stun him. Once stunned you can spam knife or pump shotgun into it until its HP reaches zero.
Honestly, there's a major overhaul to the game coming up which is why the next update has taken so long, there will be pop up messages to help understand what the boss does so its less of a guessing game, crafting was implemented along with armor and accessories to reduce damage and prevent status effects, status effects are being added. Won't go too much into it as I'm still putting it together.
I never thought of the base speed being too fast, I'll experiment with making the character slower. I've tried adding stamina to the run as well but that would make certain bosses unkillable so it got scraped.