Primarily they are looking for the overall time, not "speed". Each stop/landing also decreases their interest. Aircraft comfort factor has an effect but it is small.
So basically the bug is that the pax are not looking at overall time either.
On every route that I have with competition against a prop, when I change from prop to jet, the ratio of market share stays the same (50/50, or 33/66 if I have only one frequency, etc.), with no additional market share being captured by me, even though I now have a faster plane and kept the same pricing against the same competition.
And since jets burn more fuel, changing to jets is actually counterproductive.
Maybe in addition to a "plane to small" warning there should be a "plane too slow" warning which is based on distance, with standards that become gradually more strict as technology/date advances? Realistically, pax may not care about flying a slow prop plane 300 nm, but when they are going 3,000 nm, they should care.
And like "plane too small," the penalty can be made to kick in only when a competitor is serving the route with appropriate equipment. That would completely change the dynamics of Jet Age and avoid having people fly DC6Bs and Bristol Britannias into the 1990s or whenever sound bans come into effect. (And actually, these prop plaes are probably not that noisy compared to turbojets, although the props did not obtain formal noise certifications because they were out of use already by then.)