5G is slow
Here are a few reasons why 5G can be slow:
That there is no 5G in the first place
The criteria required to be met before a 5G icon can be displayed varies. At the minimum, phones may display a 5G icon when the setting upperLayerIndication-r15=true is loaded from the tower, and this collerates with 5G but does not guarantee that there is 5G.
That the width of 5G is small
5G can have a width as narrow as 3G's width. If the width is narrow, there aren't many waves that can carry actual data and error correction data. The width of 5G varies depending on the license the carrier has and 5G isn't always wide enough to exceed 1Gbps. If 5G were to be compared with 4G and there's the same width, 5G would only have a slightly faster maximum download speed.
That at least one of several other factors lowered the speed
Congestion, interference, throttling, etc.