On the Java, vexes are very fast when they are glowing red while they are lunging/swooping at there target, they can even lunge/swoop behind their targets. On Bedrock, they are very slow as they just bob up and down when they are about to lunge and cannot lunge/swoop behind their targets.

To reproduce
1. Go to survival
2. Spawn vex (/summon, spawn evoker or spawn vex)
3. Wait for the vex to glow red

When spawned by a command, evoker or the vex spawn egg, they are not very fast and players can escape the very quickly.