As mentioned in MCPE-55786, lingering potions have black particle effects, regardless of potion type. Unlike in MCPE-66626, MCPE-55032, & MCPE-54979, my PC was just barely able to cope with the massive number of particles being rendered; leaving into higher subchunks (Y-axis) alleviated the tremendous lag. Returning before the effect has finished results in the same intense lag.

I noticed that the radius of the lingering potion (a health II potion in the end) was 30 blocks*. This results in 900π blocks being covered by the effect, or 2827 blocks. That's a lot of particles, and yes, the math checks out. This appears to be 10 times the intended radius, which likely means someone coding the lingering effect added an extra zero. This may also resolve the reason for the above-mentioned tickets, excluding MCPE-55786, which appears to have only 2-3 times the intended radius, avoiding a crash.

*As seen in the screenshots, the radius seems to vary from 20–30 blocks, depending on distance. From the player's perspective, they escape the effect's are moving 30 blocks from the center. In the overworld & nether, potions seem to have the normal area (but still black).