From what I heard earlier, the room was free due to his high traffic in the casino.

makes sense

I'm still wondering why the security took so long to react after knowing exactly where the shooter was, before the main shooting started.

• People either panic or tunnel vision; it is likely that hotel staff were focused on medical care for the guard; they might not have even been aware that a mass shooting was taking place for some time
• 911 response was going to be slow; I expect that with so many calls coming in with conflicting info, LE suffered from information overload
• I was under the impression that it was an FBI HRT that took out the shooter; that would have been an amazing react time; does bring up the question of why the FBI was dealing with a LVMPD SWAT situation.
• I Don't expect that the other guards were eager to take on someone that put multiple shots down range already.