[monsieurbanana]

How does the number of pixels (or even monitors, for that matter), affect that much sleeping time?

Rendering a FPS game at 1080p is 2 million pixels per frame. At 60fps, that's rendering 120 million pixels per second.

What am I missing?

[bzzzt]

Detecting the monitors, negotiating the correct resolutions, setting scaling factors and window positions after coming out of sleep will take some time. Maybe macOS does monitor setup sequentially? (no idea, just got 1 big external screen that also takes a few seconds to light up - handshake speed seems to vary between monitor brands)

[bombcar]

This is almost certainly it - the screens show all sorts of weird graphical artifacts (some clearly a Retina display in "native" resolution) as it starts and loads. I assume it's having to fire up all the GPU memory, etc.