i've been setting belgranostudio.com .htaccess file, with all the expire and gzip compresion settings that I could find on the web, and my tests have really improved. I've implemented a CDN also, Cloudflare.
But this settings don't seem to affect the subdomains performance that much; everyhing is going faster, anyway.
I don't want to use a plugin like W3; if there's some other solution, I'll be glad to hear it.
Thanks everybody !