Hi TheBerbster, thank you very much for your links and advices, I will go through them, I'm sure they will help me out.
I think I can make good progresses in launching SH3, the main problem being the many mod objects present in WAC 5.01. Currently I'm trying, as suggested by Gatordad below, to see how I can improve running without Mods or/and activating fewer external decors like complex ports and bunkers that are very eye-candy but also very ressource-demanding, thus I guess the long loading time of 5 minutes per mission. I didn't try starting a campaign yet.