autodelay — spe and slowkeys — time estimation

[T U]

spe 10000;;
opt slowkeys 1

key "1234567890"

The help document says about "slowkeys":
"The speed depends on macro speed (spe)."

How can I estimate the time it will take for the keys to be pressed (time of all of them and time between each one e.g. between 2 and 3)? I have experimented with different speed values and numbers of characters but I can't figer it out.

note: The values mentioned bellow were recorded without changing the numbers of characters (10). Also, this consist a rough estimation because I was just looking at the computer's clock but (IMHO) for the rough description of the query they should be good enouph.

value — the time it takes to press the 10th key (the duration of the macro is longer than this)
1000 — 4 sec
2000 — 8 sec
3000 — 11 sec
4000 — 15 sec
10000 — 36 sec

a) The delay starts after the first key?

b) How to stop it? The Pause key doesn't do it.

c) What do I have to add at the end of this code so that the rest of the lines I'm going to write will be executed at the default speed (like if I had never used the "spe" command)?

d) If, in a macro, we want to change the speed for the keys many times but keep the time for the other commands unchanged, how should we do it? I don't mean a loop or something; I mean, for example, at some part of the code I want the keys to have a speed of 2x, at another part x, then 10x, and the last group 4x.