a) Could be. There are other factors in play for good SEO
b) Not really. Have you tried http://wpml.org/ plugin?
c) It depends on your webhost – bandwith and memory allocated, etc.
However, this forum is for BuddyPress, proceed to WP Multisite forums https://wordpress.org/support/forum/multisite and use same username and password you used here to log in there to get further assistance.