Communication speed is affected by the location and the surrounding environment (buildings and other obstacles, congestion of the line at that time of day, etc.).
Also, in some parts of Japan, it is very difficult to get a signal.
Please try moving to a different location or try at a different time of day to see if the situation improves.
Additionally, connecting many communication devices to one Pocket WIFI and using them simultaneously may temporarily slow down the communication speed.
Please let us know if this does not help.