fast-but-imprecise-scrolling

fast-but-imprecise-scrolling is a variable defined in `C source code'.
Its value is

nil

Documentation:
When non-nil, accelerate scrolling operations.
This comes into play when scrolling rapidly over previously
unfontified buffer regions. Only those portions of the buffer which
are actually going to be displayed get fontified.

Note that this optimization can cause the portion of the buffer
displayed after a scrolling operation to be somewhat inaccurate.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 25.1 of Emacs.