Repeated actions aren't always synchronous

[Chillee]

Related: microsoft/vscode#25278
#1340 may also be related

What happened:
Have a set up like

|foo
bar

Press yy to copy the foo, and then hold down p.

The foo will eventually jump past the bar, resulting in something like

...
foo
foo
bar
foo
foo
...

This is the one place that I've encountered this consistently, but considering the above bug report and the likely causes, I'm guessing this may be a more pervasive issue.

What did you expect to happen:
The bar stays at the bottom.