26.2; Strange slowness in some Info nodes

[ndame <emacsuser@freemail.hu>]

I don't know if it's a known bug or some system dependent issue
that's why I'm sending it to help.

I recently moved to emacs 26 and today I noticed a strange
scrolling slowdown in certain Info nodes.

It also occurs with emacs -q.

If I go to Elisp info and try to scroll in the main index with
page up/down then everything is fine and quick.

If I go to Lists and then to Building Lists and there try to
scroll down with page down, then the scrolling becomes slow, like
it takes one second for emacs to scroll the page after pressing
page down/up.

The problem seems to occur only in info nodes where there are
separate sections describing entities like functions in Building
Lists:

    -- Function: cons object1 object2
    This function is the most basic function for building new list
    structure.  It creates a new cons cell, making OBJECT1 the CAR,
    and OBJECT2 the CDR.  It then returns the new cons cell.  The
    arguments OBJECT1 and OBJECT2 may be any Lisp objects, but most
    often OBJECT2 is a list.

In nodes with free flow text there is no problem with scrolling, so
it's like info does something extra in entity describing nodes.

It's 26.2 on Windows taken from the official GNU site:

  GNU Emacs 26.2 (build 1, x86_64-w64-mingw32) of 2019-04-13

The problem also occurred with 26.1  I upgraded to 26.2. but it's
the same there.

Does anyone else experience this? Is this a known issue?