Finally I found a way to actually PLAY this game !!

I put this solution here because I found nothing on the internet speaking about it.

First of all I explain better what was happening, if you recognize the issue for you too:
during gameplay, when a lot of people are present or a lot of things are happening, the game start freezing for 1/2 seconds while the last sound/speak is still playing... during that time the mouse is still responsive but you can't click anywhere... also this freeze can happen once every second if there are too many monsters around, and it's impossible to play like this...
I had also the "slow gameplay" issue at fullscreen, the one that when you set the speed to maximun it's still slow, and this solution covered even that for me.

With these I can play FULL HD (fullscreen) without issues on Win 8.1, WITHOUT compatibility settings and WITHOUT command lines:

1) modify the registry key "HKCU\Software\Cyberlore\Majesty Expansion\MaxConcurrentSounds" with hexadecimal value "200" (or decimal "512")

(The default value was 0 in my case, and changing this removed totally the freezes)

2) modify the file "%DOCUMENTS%\My Games\MajestyHD\MajXPrefs" like this (changing it for your system specs):

<DataGroups>
<PixWidth>1920</PixWidth>
<PixHeight>1080</PixHeight>
<PixDepth>16</PixDepth>
<Windowed>0</Windowed>
<DebugFlags>0</DebugFlags>
<VSync>1</VSync>
<BlitMode>1</BlitMode>
<IntroVideo>0</IntroVideo>
</DataGroups>

(This skip also the starting cinematic, that it's the main reason of the slow gameplay... you can also watch it from the main menu, but still after that you will play slow... so don't watch it if you want to play the game at a decent speed...)

EDIT:

updated after patch "1.5.1.1 HOTFIX1"

- Now it is better to leave color depth at 16 as it is by default (with 32 the intro has wrong colors, and also the game seems less smoother)

- The game speed seems not anymore impacted by playing the intro video.


EDIT:

as of patch "1.5.2.28" I find no more issues...
the gameplay speed seems correct without lags and the intro video gives no problems too :)