java 8 update 171-b11
Edit: The beacon isn't relevant. It's just a red herring, caused by my bias.
Just updated my 1.13 world that has existed in multiple versions. (1.9, 1.10, 1.12.2, 1.13-pre2, 1.13 and maybe others).
Under the effect of the 6 beacon buffs I enter the nether. Running as much as possible, so I get the most distance out of the beacon buffs. I keep the speed increase, even while crouching, even while not sprinting. There is no difference between walking and sprinting either. I seem to be sprinting. Also, the eating sound has sped up, but only when the speed thing happens, so... no separate issue, if that's okay.
Under the effect of the 6 beacon buffs I enter the nether. This time I stand still and wait. All is normal.
Testing in creative default world made specifically to test this bug:
Side effects include:
-buff timer counting twice as fast, buffs not expiring, being stuck at 0 when going out of range of the beacons under unknown, slightly different, but nonetheless common conditions.
inability to fly in creative when switching from survival while under the effects of this bug Actually just required pressing space faster, because: jumps also go faster
-dayLightCycle being sped up
-redstone repeaters working faster. Pistons potentially as well. Sticky piston doesn't tire out with 2 repeaters set to 1, even though the game sometimes catches up (doing 10 or so extensions in a lot less than a second)
-falling speed is at 100 blocks/second
It feels like I'm at speed 25 or 50 just from running around the general area in which I built the beacon. Original issue now only occured in the overworld for reasons unknown.
Relogging properly updates the speed back to normal.
Steps to reproduce:
1. Build a 4-level pyramid with 6 beacons on top
2. activate each of the 6 buffs
3. Build a nether portal next to the beacon pyramid
4. While under the effects, go through the nether portal
5. Wait for the buffs to reach 0 seconds remaining (they'll stay at 0 forever at this point)
6. Enter the overworld