Currently our "smpl SIMs" work on AT&T and T-Mobile, and is determined by the device when it rebooted or reset based on reception to the nearest tower. As with any cellular system, speeds are variable based on a number of factors from congestion, time of day, reception to you, and other environmental factors.
As an IoT certified device, our speeds start slower and ramp up faster as your download/upload more. The purpose of this is to prioritize Latency (that's the quality of the connection) and allow for better services like Voice over IP, Voice over LTE, and other critical services. We do not limit on speed, but may vary by area.