Why is there a delay when typing certain characters with xdotool in 18.04?

Using xdotool type or xdotool key with certain characters makes Gnome Shell lock up. For example typing a single slash with xdotool type / or xdotool key slash takes a second or two, but for something like 23 slashes (i.e. xdotool type ///////////////////////) it's more than 25 seconds.

Of all the printable ASCII characters, only these cause the problem:

• slash /
• backslash \
• bar |
• bracketleft [
• bracketright ]
• braceleft {
• braceright }

Outside ASCII, I've only tested çé°†, but they also cause the problem.

Why does this happen and how can I fix it?

More details

I never had this problem in Unity on 14.04.

There is a small delay when typing other characters, but only up to a second, even for long strings.

Any timing I've done in the terminal has come back normal, like time xdotool type ... or t=$SECONDS; xdotool ...; echo $((SECONDS-t)), so it seems to be a problem on Gnome Shell's side.

I've confirmed $XDG_SESSION_TYPE is x11.