Check this tweet from Speedsmart.net. In particular, take a look at this image:

For each carrier, the top part of the image is the download (top) and upload (bottom) speed of the iPhone XS and iPhone XS Max (I read this as, both models have identical speeds).

Below that set of bars is a smaller set which show the speeds for the iPhone X. These are tremendous speed increases. I’d love to see some verification of these tests from other iPhone X and iPhone XS users, but if these results are accurate, and mobile speed is an issue for you, that’s a compelling reason to upgrade.