Your movement speed is affected by the weapon you're carrying. It's currently unknown if each weapon has its own weight, but there are at least three weight classes:

Light: Unarmed Grenades Pistol

Medium: Submachinegun Assault Rifle Shotgun ASP Rifle Penetrator

Heavy: Particle Weapon Rocket Launcher Repeating Cannon

Credit goes to: https://kb.speeddemosarchive.com/F.E.A.R.