Delay after typing shift-o to insert a new line between brackets

Question

I don't know why it is happening.

Suppose I've got a code like this (let x be our cursor):

int main() {x

and then

int main() {
}x

Now I would like to enter the normal mode and type shift + o to insert a new line above }.

However this is what happens:

int main() {
}Ox

and after a while it changes to

int main() {
    x
}

Here is a video where I was able to reproduce this:

Vim hangs for a second when typing shift-o after inserting curly brackets on YouTube

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Side note: could you please suggest me better tagging for this question? I've got no idea how to tag it.