According to the volumetric fog thread:
Temporal Reprojection
The volume textures used by Volumetric
Fog are relatively low resolution and
aligned to the camera frustum.
Volumetric Fog uses a heavy temporal
reprojection filter with a different
sub-voxel jitter each frame to smooth
out the aliasing. As a side effect,
fast changing lights like flashlights
and muzzle flashes leave lighting
trails. Disable volumetric fog
contribution on these lights with
‘Volumetric Scattering Intensity’ set
to 0.
Besides setting it to 0 is there any other way (even if it means changing some values on the engine’s source code) to increase the update frequency to minimize the trails?