It isn't the filth that comes out behind the pellet that is the problem. It is the residue from the previous shot coming out in front of the pellet that gets compressed into a tight wad and may trip the first optic.

Even the smokiest old springer full of Easy Start and diesel will give one good shot from cold. It's the second shot that will read slow, think smoky barrel residue.

A pre-charge pneumatic drenched in pellet lube can do it.

A CO2 gun using oily cartridges can do it, also makes lousy fizzy pop if you have a Soda Stream.

If you can see a puff of smoke then the cb-625 may also see it to.

First thing to try is moving the cb-625 a tadge forwards because a puff of smoke has poor cohesive qualities at high speed, unless it can form a ring.

Failing that, as was said, shoot through a piece of paper or stick masking tape over the cb-625 entrance. Something the pellet has to pierce, make sure the pellet arrives first at the optics. If this makes not one jot of difference then you do not have the problem.