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Yes.  My brain has been in local SEO mode this week, and my first question should have been to ask about site size.  If the file is huge, then it's a huge file.  Huge and fast seldom go hand in hand.

How many pages are you redirecting?

The ultimate source for testing what affects page load in terms of assets is here:  http://tools.pingdom.com/fpt/

The size of the htaccess file shouldn't really affect a page's load speed.  With a 301 redirect, if someone goes to the new page, I don't see why it would load more slowly.  If you went to the old page, you're adding on the time of the redirect.  Since hopefully the old page shouldn't be rendering anything, it should be near instant.

Yes.  My brain has been in local SEO mode this week, and my first question should have been to ask about site size.  If the file is huge, then it's a huge file.  Huge and fast seldom go hand in hand.