On an unlimited plan, the usual reason is the daily full-speed amount. Once a day passes it, the connection carries on at the slower speed printed on that plan until the next day begins, when full speed returns.
The other reasons are local: coverage where you are standing, the time of day, and how busy the network is. We buy access to local networks and cannot make them faster than they are.
Check which network the phone is on, and try picking another by hand. A different operator in the same place can be a different experience.