Vim - Source of delayed display effects (e.g. when pasting data)?

There's something odd that I observed over the years with Vim...

When inserting a new empty line (with 'O') or pasting the contents
of a copy/delete operation (with 'P') there's (for a short moment)
first an 'O' and a 'P' respectively seen on the target line before
the desired text contents are placed there.

(That happens occasionally and I cannot force reproduction of this
effect).

It's not a problem, just a slight distraction, but I thought I ask
(out of curiosity) whether someone has an idea what could be the
reason for that effect.

Janis