It'd be simple enough to have a few body classes (e.g. "animations-slow, animations-default, animations-fast"), and have it be a user-enabled setting somewhere

It'd be simple enough to have a few body classes (e.g. "animations-slow, animations-default, animations-fast"), and have it be a user-enabled setting somewhere that simply swaps those classes based on cookie. Similar to how some table-density padding / spacing options are constructed.