Is the person on the base plan or the plus plan? If on the base plan it is likely due to traffic/congestion on the towers. If on the plus plan has the person used up the 50GB of premium data, because then you would be deprioritized again and like the base plan it would be due to heavy traffic/congestion on the towers in that area. One thing to try would be to go into settings and find reset network settings(not factory reset) and reset the network. Keep in mind this will reset hotspot name and password and if the reset include Bluetooth it will forget or clear any previously linked devices so they will have to be linked again as they won't link automatically next time.
I am on the base plan and since school has been out for the summer I have hit parts of the afternoon where my data speeds are at a crawl <1Mbps due to the traffic. Not much one can due but I just live with it and wait it out. I ran a test about 30 minutes ago and it was barely registering 1Mbps, this is using the Ookla speed test app. Also, if you use fast.com for the speed test that is for video speeds and Visible limits video streaming to around 2Mbps, this would be on phone and on any device connected to the hotspot.
If you think there is an issue then you would have to contact Visible by chat or message them on one of their social media platforms, Facebook, Twitter, Instagram and even on their support page on Reddit. They may be able to re-provision the SIM or do some background work and possibly improve it.
