I recently took one apart that I bought used (thanks for the good instructions btw), because the cylinder and cyclone were covered in fine white dust, which required washing to be removed. Probably plaster dust or similar. I wanted to make sure it was all cleaned out before further use. Luckily, the inside of the motor looked fine and not much dust in the outwards filter, which either means the cyclone is indeed very efficient or the HEPA filter is not very HE.
Nevertheless, since I will be using that in my workshop, for caution I decided to put a pre-filter in, which is not very difficult. I used a disc from a polishing pad for parquet floors, which is quite coarse grain (see picture against the light) but still keeps a lot of stuff out. Cheap and easily washed or replaced.
The joint between the cylinder and the body entry is flexible, so squeezing the pad in between works fine, suction on the business end does not feel any lower than before and the motor does not sound any different either. So it does not seem to block the air flow too much
Otherwise, I am happy with the Dyson, it does a great job.
After, with pad inserted:
The pad material held in front of an LED tube: