Many drivers might found that shifting occupied a part of speed up process and lower the acceleration. But there're several ways that can reduce or avoid that, Using the Auto/Manual shifting function of ETS2/ATS.
Now I First
define Auto/Manual shifting switch as '1'.
define Force auto shift up as '2'.
define Force auto shift down as '3'.
And I'll provide 3 ways quick shifting...
1. Short latency shifting skill
First set "Adaptive automatic transmission" to "High power mode", you can find it at Options/Controls.
When engine rpm comes up "OVER" 2000, Double click '1' as fast as you can,
than you can get a shorter shifting latency.
When shift down, rpm should lower than 1000 to do it.
2. No latency shifting skill
First set "Adaptive automatic transmission" to "High power mode".
Switch to auto mode.
When rpm comes "CLOSE TO" 2000, click '1' and '2' at the same time, (If over, it might jump gear and you'll get full latency)
you'll shift up with no latency.
When shift down, first switch to auto mode, and click '1' '3'.
3. No latency shifting skill with macros.
You should mind that shifting up at full throttle on three ways.
That's all, I've made a sample video.
Hope it helps you getting a quicker shifting speed.