The speed difference is more so between the 256 GB models and the higher capacity options. That’s because, as dmcloud says, the 256 GB model uses just one flash module whereas 512 GB models and above use multiple.
Its sort of like trying to channel traffic down a lane - the 256 GB has just one lane for traffic to go down, whereas the 512 GB (for example) has two lanes meaning traffic can travel faster and more efficiently. How this transfers into real world usage depends on the user.
If you have lots of data, the higher storage option is typically best. You may still notice slow down if you’re running out of hard disk space, regardless of the capacity.