Thanks! Okay maybe... However I'm not sure this delay comes from the link between camera and Pi, as if I go on Indi control pannel of DSLR and check "Upload local" instead of "Upload client" it takes much less time (about 5 secondes). But this setting is always overwritten by Ekos for the next picture.
It is indeed not a problem when I shoot on my "big" setup (APO+AZEQ6), on which I autoguide and take long exposures + dithering after each frame. But it becomes a problem with my light/portable setup (lense+SA), on which I dont autoguide, dont dither, and take short exposures (1 minutes / 1 minutes and half max).