For Unlimited SIM card plans, you will have unlimited high-speed data transfer.
For Pocket WiFi plans, the data is unlimited in terms of low-speed. The Pocket WiFi will only turn into low-speed mode if you have finished the daily allowance (300MB, 700MB, 3,000MB).
※To manage network resources, unlimited data plans are managed by a policy called fair usage. In wireless internet services, unlimited data means you will not be disconnected based on your usage or amount of data transferred.