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With such fast load speeds there is no way you're running into trouble on that front. It's far more likely that it's a quality issue, especially if you believe there are a number of poorly generated pages.

Are there any discrepancies between the number of pages you're seeing on Google and Bing via the site:domain.com query, and the number of pages in the index as shown in Webmaster Tools? It's always possible that some other form of indexing issue is at play.

This is an extremely subjective question, and I think you need to work it backwards.

Keep in mind that exact match search volume tends to be the most accurate (when comparing to impression data in GWMT.)

Use data about your site's existing conversion rate, and see what your site's typical CTR happens to be for various positions by looking into webmaster tools data. Once you have that you can say something like, "well, my site converts at 3%, and my CTR rate in position 1 tends to be 20%, so I need at least 170 impressions to generate 1 conversion."

Then you factor in the value of a conversion, and the estimated cost to get you ranking in the top position, and see if the keyword is worth the effort.

If a conversion is worth $5k, and your site converts at 10%, then even a keyword with 50 searches a month might be worth targeting