Thanks guys for your suggestions.
I'll try tonight to play with the value of this "Shrink TMC grid squares" option. I'm not sure to understand what that option means but I really hope it is the key to explain my extreme slowness.
In my case I need a gain much bigger than you had Lucky1960 because ~x3 faster still means ~15 hours for a single level 12 tile (zoom level 18). But I set to 0 (instead of 0.01) I hope the gain will be even bigger..