From Halopedia, the Halo wiki
A speedrun is an attempt by players to finish any campaign level as fast as possible. This is done by having good understanding of the game's gameplay mechanics and exploiting these mechanics. Common methods in a Halo speedrun is by using explosive jumping technique, careful selection of weapons and bypassing as many engagements as possible, sometimes getting outside the map to do so. Allies may also be killed to skip dialogue. Halo speedruns are typically run on either Easy or Legendary, with each difficulty being a separate category. A subcategory under Legendary, L.A.S.O. (Legendary All Skulls On) requires the runner to activate all skulls while playing on Legendary. L.A.S.O. typically requires different routing from that of a no skulls Legendary run.
Due to performance limitations between a controller and mouse and keyboard (MKB), runners may prefer running on PC through Halo: The Master Chief Collection if possible.
Note: Records posted below may be out-of-date, please refer to haloruns.com for all up-to-date records.
Halo: Combat Evolved
There are two variations to skip the level:
Note: After performing this trick, NO checkpoints will be given (i.e. if you die, you go back to the bridge).
By using a trick called "flood bumping", it is possible to skip many of the doors that normally would require waiting for up to 90 seconds.
Through a bit of wiggling, it is possible to get a Ghost or Banshee past the destroyed door.
Using another flood bump, most of the level, including not having to go down onto the surface and back up, is skipped.
During the later part of the final Warthog run where Cortana notifies the Chief of a gap ahead, it is actually safer to do a 360, as going straight down can force you out of the vehicle and possibly killing you.
A major trick, known as sword flying, depends on the original Halo 2 version 1.0 and the game running at 30fps. Therefore, Halo 2 Anniversary is considered a different category.
By using a grenade jump, the player can reach out of bounds and walk on the roofs on many buildings, skipping the first part of the level.
By exploiting the Permanent invisibility glitch, the level is a lot easier to traverse.
You can end the level early by jumping out of bounds and navigating to a certain point.
Halo 3: ODST
Halo 5: Guardians