rng-validate-chunk-size

rng-validate-chunk-size is a variable defined in `rng-valid.el'.
Its value is

8000

Documentation:
Number of characters in a RELAX NG validation chunk.
A validation chunk will be the smallest chunk that is at least this
size and ends with a tag. After validating a chunk, validation will
continue only if Emacs is still idle.

You can customize this variable.