Some combined factors such as coverage, location, network traffic and the type of data being transmitted have can have an impact on your data speeds.
You can run a Network Diagnosis test in the Circles.Life app > Help > Network Diagnosis.
If you find that you have slow speed all the time, regardless of location, please check out this article for help.
If you're subscribed to the UNLIMITED Data Add-on, you may have hit the quota for 4G+ speeds. Check your dashboard to see if your data has been throttled. To get back to 4G+ speeds, just purchase a Boost, turn on/off airplane mode and you'll be back up to high speeds!