Okay, did a bunch of testing using the modified project you uploaded (helping_daniele1.rar). I included a bunch of screenshots just to make absolutely sure that we're testing the same thing. I've circled and highlighted all relevant data. I also added a bunch of nonsense MIDI to all the tracks to give the CPU something to crunch on.
Originally Posted by: DANIELE-ES 
In your project, the updated version (but the same would be with the original one) if I disable feedback routing I get a RT at 0%, if I enable it I get a RT at 12-13%. Not too much in general but too much if you think about bigger templates.
Using exactly the settings you uploaded:
With feedback routing enabled:
With feedback routing disabled:
- RT CPU was 4.9% while idling. Screenshot.
- RT CPU was 8.2% during playback. No screenshot here since it would be redundant.
Originally Posted by: DANIELE-ES 
this is because even if I put offline my FXs sends remain active and there is where the problem is.
With the VEP FX instances bypassed but the sends still active:
Originally Posted by: DANIELE-ES 
...If I mute my tracks I see a big improvement...
...if I disable it or if I delete all sends RT drop down from 50-60% to 10%...
With the FX online, but all instrument tracks muted and all sends bypassed:
And just to be thorough, here is a screenshot of the VEP7 window with the instances. They're all active and duplicates of each other, and you can see in the second screenshot I uploaded that they are all playing as they should.
Originally Posted by: DANIELE-ES 
This is not a VSL fault obviously but untill the Reaper Devs will go to work on this (and who knows when) VEP is useless in these conditions.
Except, for whatever head-scratching reason, it's not useless on my machine. Using the exact project you uploaded, I get completely different RT CPU readouts (as you can see in the screenshots), none of which even get to the 12-13% you're getting while idling even during playback, despite you having a faster CPU (and all these tests were done with a bunch of other apps running in the background). For you, disabling the sends dramatically increases performance, but for me, it's the opposite. For you, disabling the FX doesn't do much of anything, but for me, that (logically) reduces the CPU overhead.
WTF?!?!?!??!
This absolutely convinces me that there is something specific to your set-up/hardware/whatever that's making feedback routing buggy and CPU intensive. You're right, the REAPER devs should know about this; thank you for alerting them.
I did think of one thing that might help:
Did you test to see if youre CPU is actually multi-threading? Using CoreTemp I monitored the idling CPU usage to make sure all threads were being utilized. Screenshot. If only one core is actually being utilized, the issues you're experiencing would make a lot more sense. Worth a check, I think...
I'm afraid that's the best I can offer...if I think of anything else I'll let you know.
- Sam