The Latin genre has far more variety in its song written arrangements than probably all other dances combined.
One needs to know that, first " Rumba " is an umbrella term that encompasses several of the " Son " rhythms, namely.. Guaguanco, Guaracha, Montuno, Guajira and Son Montuno .
This allows for speed changes as in Rumba/Bolero ( its correct name ) thru to Guaracha ( a very fast form and is an off shoot of Danzon ,the original form ).
Going back to merengue , again, it depends upon the country in which the music is recorded . Even the Domincans music has a wide variety of tempi, some extremely fast ( Dominicanos LUV that ! ).
So, changing speeds within a song does happen in ALL dances ( we syncop. where there is NO syncop. in some dances ! ) .. more to the point, is knowing WHERE and WHEN to change .
PS.. halftime Rumba could be a Bolero?