Treadmills are very handy for speed work, particularly popular among athletes who are short on available training time, but there has to be a balance between indoor sessions and outdoor sessions if you truly want to see gains on the open road.
The limitation behind a treadmill is that you are really just trying to keep up instead of propelling your body mass forward. You do work very similar muscles, but it is harder to gain real-world speed. A treadmill is excellent for helping you work on quick leg turnover, and it can be a great tool when you are lacking motivation to push the pace outside, but if you really want to be able to maintain that leg speed during a race you need some outside time on a 400m track, or smooth open road with no distractions.
That said, getting inside for some speed workouts can limit risk of injury as in general, most treadmills offer more shock absorption than the open road and you can easily fine tune your pace.
One thing, many people surprisingly find, is that it almost always seems easier to hold that given pace OUTSIDE, than it is on a treadmill. You would think the opposite would be true, but it can be a lot harder to hit the machine’s pace readout, than to hold that same pace on the track. This again is most likely down to your body mass being in motion outside, helping you maintain speed.
Variety is the key for a good treadmill workout (mix up different timed sections at differing pace) and take heart from the progression between sessions and not necessarily the comparison with data from outside sessions.