should the main site of a large multisite have its own blog and blog

so a question on speed when it comes to wordpress and such.... if I have a multisite that I hope anyway will get big... how big I dunno so assume huge just in case im blessed with that problem.

Anyway Ive ended up messing with the php files of most the site to try to hard code most of it in except what needs to be dynamic to increase speed and efficiency... my primary site must have the signup obviously, I figured id leave the primary news blog on that site as well.

Now the question I have from a performance standpoint... assuming that both of these wouldnt be on the front webpage, but in a subpage somewhere on the main site would it be better to go ahead and leave the forums on that same site... Im thinkin it would be best that way if for no other reason then its easiest to mess with the forum widgets and such.... but Im not sure if I would take a performance hit.

The other question is blog... Besides the news I plan on having a blog with relevant info for my network of sites....not to mention another piece of content out there to get indexed for google. Would I see a performance hit there? or is that fine to just leave on my primary site.

Primary site as in example.com's blog or would I be better with blog.example.com for the blogs site or forum.example.com for the forums

thanks