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