We recently did a Caribbean cruise (Ruby Princess) out of Galveston and found the Medallionnet WiFi internet access rather slow and spotty. The best time seemed to be really early in the morning before the rest of the ship was awake.
As other's mentioned, in port cell data (LTE and some 5G) was much faster and more reliable when in port. Also, i found using the ships computers in the internet cafe to be faster and more reliable than the WiFi, enough to stream video even. My only guess would be the internet cafe desktops are hardwired ethernet connections vs the ships WiFI that has to fight with all the metal walls.