All we can assume about a 5G smartphone from Samsung is that it'll include the 5G-compatible model of Qualcomm's Snapdragon 855 mobile chip. What kind of speeds and performance we can realistically expect from mobile 5G vary greatly. It's generally expected that mobile 5G networks will be significantly faster than the current 4G LTE networks we currently use to stream data on our smartphones today.
We'll have to wait and see for a true 5G smartphone connected to a 5G mobile network to find out just how much better 5G will be than what we have now.