First off what app or website are you using to test speeds? If using fast that is video speeds, try speed test app by Ookla. If you are on the base plan you being deprioritized due to congestion or heavy traffic on the towers in your area because Visible is treated as an MVNO and doesn't get the priority or a Verizon post paid customer. Another issue is if you have a weak signal. If on 5G try going into settings and have it use 4G LTE only and see if that helps, not all phones can do this and not sure how to change, just have read some phones can do this.
You can try Visible+ and get 50GB of premium data and see if that improves speeds but once you use the 50GB you will be deprioritized again.
Edit: You could also try contacting customer service and see if they can (re)provision your SIM and see if that helps. When you contact them explain that you have slow speeds and ask if they can see what they can do on their end to fix the issue. I am not sure exactly what they do but have had it done but improvements are nil.
