hi-lock-highlight-range

hi-lock-highlight-range is a variable defined in `hi-lock.el'.
Its value is

200000

Documentation:
Size of area highlighted by hi-lock when font-lock not active.
Font-lock is not active in buffers that do their own highlighting,
such as the buffer created by `list-colors-display'. In those buffers
hi-lock patterns will only be applied over a range of
`hi-lock-highlight-range' characters. If font-lock is active then
highlighting will be applied throughout the buffer.

You can customize this variable.