The cause can never be established with certainty at a distance, but three things regularly come into question. First, residues: a pen that has stood unused for a long time or has been written with the same ink for years builds up deposits in the feed that slow the flow. The remedy is a thorough flush with lukewarm water, overnight if need be, done regularly and not only once things start to stick. Second, the nib: if it does not sit properly on the feed or is slightly bent, the two tines are no longer level with each other and the pen skips as the stroke begins. Third, the ink: flow characteristics differ considerably between brands, and simply changing ink can cure sluggish writing.
As a rule of thumb: if flushing brings only brief improvement and the flow then deteriorates again, the cause lies with the nib and feed rather than with dirt. Straightening a nib belongs in expert hands, because the slit between the tines can hardly be judged with the naked eye and clumsy attempts damage the iridium tip. Very broad nibs on older pens also lay down a great deal of ink by their nature, and it dries slowly; that is a property of the nib and not a defect, but it makes writing at speed hard going.