The Navier-Stockes equations derivation assume that they are supported in a continuum domain. You can know more about that size based on the Knudsen number (https://en.m.wikipedia.org/wiki/Knudsen_number#:~:text=The%20Knudsen%20number%20(Kn)%20is,a%20body%20in%20a%20fluid.)
Hello Grace! I am not aware of what you are trying to solve, but this is a very small domain where you find a velocity considerably high (for the domaine size). Look at how much time a particle need to take from the entry to the outlet of your domain. It is a very small time interval. Furthermore, this domain is discretized, so the cell elements may be very close to the minimum size of the assumption of a continuum.
Relatively to the convergence, you should look to your Courant number (you may also find the name CFL number). This may give you a clew about the reasons to the slow convergence, but I also suspect it is related with my previous points.
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