Some combined factors such as coverage, location, network traffic and the type of data being transmitted have an impact on the speed.
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 check 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!