In Vim, why is temporary Normal Mode (Ctrl-O from Insert Mode) executing commands much slower?

Question

Usually, when I want to undo changes while editing text in Vim, I have to:

1. Exit Insert mode with Esc
2. Press u to undo changes
3. Enter into Insert mode with various commands (i, o, etc...)

Now, I am just a beginner, but I understand that while in Insert mode, you can temporarily escape into Normal mode for one command by using Ctrl-O.

Knowing this, when I want to undo changes in text, I figured: why not just escape into temporary Normal mode and press u and not have to deal with all the shenanigans of Esc and switching between modes?

So I did the following:

1. Escape into temporary Normal mode for one command, using Ctrl-O
2. Press u to undo changes
3. I am automatically in Insert mode and continue editing.

It works... except it's VERY, VERY. slow. The cursor would be immediately placed at the position after the undo is executed, but the changes in the text aren't displayed until after ~2 seconds. That's a very long time.

I thought it was just my computer running slowly for some reason but I did the old-school way and the undo was executed instantly. Granted, this isn't the case for all commands. When I try to dd and delete the whole line, it works just as expected - instantly.

So my question is: why? Is temporary Normal mode not the same as the Normal mode one would enter from pressing Esc? Does it have anything to do with buffers, or swap files, or whatever is going on behind the scenes? Is it the way undo is implemented in the Vim editor?

Answer 1

It would appear that Vim is waiting for a possible second keypress, i.e., the motion to the command (e.g., dw, which executes normally). Obviously the u command doesn't have a motion, but double tapping uu does execute the undo quickly, it just then inserts the next u into the document. I suspect this is a quirk of how the 'temporary normal mode' decides when to return to 'insert mode'.