Thanks to curaga and juanito.
a) correct: in Xorg76, the grafics.. extension has to be loaded. Time saving at boot approx 13s
b) in xorg76 the firmware.tcz at boot only make it slower by 2s (I could not see any advantage)
c) I had a look again at xorg74.
by looking at my screen specs I had in the xorg conf file
.... # HorizSync 27-96
# VertRefresh 50-160
and looking at the xorg76 log file
.... Default mode "1024x768": 65.0 MHz, 48.4 kHz, 60.0 Hz
Modeline "1024x768"x60.0 65.00 1024 1048 1184 1344 768 771 777 806 -hsync -vsync (48.4 kHz)
and looking at the xorg74 log file, I saw the xorg76 scanned automatically a larger frequency than xorg74.
so, I activated the parameters
.... HorizSync 27-96
VertRefresh 50-160
and it goes automaticcaly to a very large pixel (too large).
so, I reduced the scanning area in xorg.conf with
HorizSync 45-50
VertRefresh 55-65
(according some frequency informations which are in the xorg76 log file "Default mode "1024x768": 65.0 MHz, 48.4 kHz, 60.0 Hz")
and now, it goes automatically into 1024x768 for both xorg74 and 7.6.
Good.
I stay with xorg74: less RAM ressources and same boot time than xorg76 (no firmware.tcz loaded).
Boot time: from the "return" key activation in the grub menue, till the background appear on the screen: 51s.
Info: I did not test xorg76 without xorg.conf file because I remember xorg74 did not start without xorg.conf file.