As of a few days ago, the fastest anyone has beat the original Halo: Combat Evolved is in one hour and 38 minutes -- it's a new world record held by Andrew "goatrope" Halabourda, and you can watch him nail it here.
To be exact, the run takes 1:38:57. The previous world record for this run was at 1:41:15, and it was also held by Halabourda. This new run shaves a few minutes off -- it might not seem like much, but sometimes that makes all the difference in a speedrun.
If you're curious about his technique, Halabourda has written something on glitches that affect routes, as well as something on when combat in the game affects his time. It's fascinating to watch this knowledge come together in the actual run, as he knows exactly where enemies will be and how to get through them as fast as possible.
You can also check out other Halo franchise records here -- this leaderboard includes legendary and easy runs. Alternatively, you can peruse world records for individual levels in the Halo franchise here.