Slow is not the same as broken, and the fix depends on which one you have.
1. Check whether you have hit a fair use limit
If you have used a lot today on an unlimited plan, your speed is reduced deliberately until the allowance resets. Check today's usage in the app. See "Fair use and speed limits on your plan".
2. Check what is left on your plan
Some plans slow down rather than stop when the data runs out. See "Check how much data you have left".
3. Check where you are
Speed is whatever the local network gives you at that spot. Inside a thick-walled building, on a busy train, or in a crowded venue, everyone around you is slow too. Step outside and retest.
4. Try another network
If your plan can use more than one local network, the one you are on may be congested. See "No service, or connected to the wrong network".
5. Turn off your VPN
A VPN routing your traffic through another country adds delay and often cuts throughput substantially. Test with it off.
6. Restart your phone
It re-attaches to the network, which can move you off a poor cell.
Then measure it
Run a speed test with the VPN off, note the figure, the time, and where you were. If it is genuinely poor across several locations, send us that with your ICCID and destination. Measurements from more than one spot are what let us tell a network problem from a local one.