There are a few block updates where there is supposed to be a specific delay. These are liquids flowing and observers giving a pulse. However, the actual delay can be shortened by an adjacent block update, assuming both are in the same chunk.

How to reproduce

1. Place 2 rows of solid blocks, with a 1-wide gap in between. Make sure the entire construction is in 1 chunk.
2. On top of each row, place observers, each facing the nearest endpoint. This gives an observer clock with an additional observer on each end.
3. Create a third "extended observer clock above the gap between the first 2.
4. Fill the gap with lava.
5. Break one of the observers on the side. I expected there to be a delay before the lava flows, since lava is slow outside of the Nether. In reality, the lava fills the observer's space almost immediately, but it then flows at more normal speed.

Impacts
Like MCPE-15793, this could break the timing of certain contraptions, especially if observers are affected by all block updates. It can also be exploited to make water and lava flow super fast, such as in Navynexus's basalt farm.