Speedrunners usually work on perfecting their craft through countless tries and it was a similar case with 4shockblast, who managed to break a 20-year-old record in DOOM's first level, Hangar, after a year of trying to get the perfect run.
For anyone claiming Sekiro is too hard and that needs a lower difficulty setting, how does beating a 20-year-old speedrunning nine-second-long record sound? Yes, the speedrunning record for DOOM's Hangar level was nine seconds for 20 years and no one could beat it.
The trick with DOOM's speedrunning is that the measuring units adopted by the community only counts seconds, rounded down regardless of whether it was closer to the next or previous second. For example, if a run took 9.57 seconds, it would be rounded down to nine seconds even though it is closer to 10.
A speedrunner named Looper came close in 2018, when he made a near-perfect run only to mess up with the last room's door, losing precious time and finishing with a nine-second run. Later analysis proved he would have scored 8.97 seconds and would have broken the record, had the dreadful door not stood in his way.
One year and 50,000 attempts later, another speedrunner, 4shockblast, managed to break the record with the help of Looper. They would to keep trying over that period, in order to master their zig-zag paths as well as door timing as to not lose a few frames that would cost them the record.
What could look as flawless to an untrained eye, looked like a result of a sequence of lucky RNG circumstances where the undead marines in the zig-zag section were well placed and even shot the player several times in the back, propelling the character's speed, resulting in a run that finally broke the record.