Yeah so again your asking the server to constantly track who is entering the "area" within 427 which is actually worse bc that player will be moving having the server to update that every few ticks. The problem with fire dmg isnt the dmg itself its the way gmod works it, it starts a new "fire source" at ervery player whos on fire.
Best way to do it is have 427 look at someone and if in range etc a fire "effect" spawns on the players pos while also taking away 10+ hp every second etc over a set time.