Wow guys, really thank you both reddeercity and masc for your fantastic informations!
@reddeercity: Your system blows my mind: very smart! And FreeNAS is very interesting!!! Thanks also for the tip on raid5 vs raid6. Surely my current-project-RAID is slowed down by both the fact that it's full as an egg (48TB in raid6 is 36TB and I have more than 33TB of data inside...) and the configuration raid6 itself. I wanted raid6 because for this project I was terrified of losing data, and this indirectly is also one of the reasons I'm avoiding mlvfs: I want to do my proxy-edit (and the future DNG-edit) without the external-raid always turned on (for extending its life, and avoiding the noise of its fans.

The proxies (now) and the definitive selected DNGs (future) will be placed onto an internal 8TB 4-NVMEs raid-0 (HPT with 4x EVO-2TB drives) that is my secret weapon

together with the two Radeon VII GPUs (I've tested grading uncompressed 14bit DNGs with a bunch of nodes, temporal denoise, effects... and the preview in Davinci goes always in real-time! Like a boss

@masc: yes, I was surprised by the “limit” of about 220 long clips before the uploading slows-down. It's strange. Anyway, that's what I did to make the job:
1. I uploaded “packs” of 220 long clips in MLVApp (it took me around 2 minutes per-pack) and I simply created the MAPPs (more than 30 minutes per pack, so it has been a looong 12-hours work-day);
2. I opened 10 instances of MLVApp and I uploaded 334 long clips into each instance (I have a total of 3340 clips to be converted);
3. With all the instances standing, I ran the first just for letting it recap all the clips, and once it started the actual conversion I aborted;
4. I repeated point 3 with every other instance;
5. After all the recaps were done, I ran all the instances together (Activity Monitor says 'round 160% of CPU for each instance).
Now my sweet hackintosh is working... alone... in the darkness of its tiny bedroom,
dreaming (perhaps) a magic lantern. We'll meet again in three days. Good night
