What I’m really interested in is for a given N1, is the resultant thrust less?
For the large by-pass machines the dirt, grundge builds up on the slow surface speed part of the fan which is the inner concave surface nearest the blade root and directly in line with the compressor inlet. This suggests that of it that gets sucked in at low taxi speeds will also partly accumulate behind the fan as well.
Obviously at higher fan speeds under power most of this will be centrifuged out of the fan discharge.
Meanwhile any accumulation on the working parts of the blade will affect the mass flow rate through the engine for a given RPM.
The control functions for the engines may vary between different types and add more fuel (higher EGT) and/or increase the RPM or hold RPM and decrease flow rate that then equals less thrust
Not a big deal unless you are paying the end of the month fuel bill on a large fleet
Now some types of grundge (fine sand/dust and even lots of rain per month) will actually erode/wear the fine leading edges of blades. This could also affect the engine cycle and in some cases the fan blades themselves. It's all in the accumulated rates and time on wing.
Manufacturer's Service Bulletins should cover any concerns