Deprioritization is handled at the cell site and is per-sector/per-band. It's handled on the radio layer and what usually appears to happen is the network will only allocate you a few physical resource blocks. So speeds will vary based on what modulation your device can get away with and how busy things are. The only 'official' post on the topic I can think of: https://newsroom.sprint.com/protecting-the-97.htm

If you live in a populated city or need to go through an airport with a phone subject to deprio you're going to have a bad time with speeds.

If you're like me and live on a mountain surrounded by trees with a line of sight to the tower down the hill, deprio isn't going to matter.