If you haven't subscribed to 5G already, the 5G signal indicates that your SIM and device is 5G-ready and that you are in a location that has 5G coverage. This means that you're still currently using data on 4G speeds.
If you have subscribed and are still facing speed issues, it could be due to network coverage based on your location or the amount of people using the 5G network in one given location.