Improving a website’s SEO and accessibility

Your team has created a blog site based on ASP.NET MVC. The site is not ranking well in Bing searches. Consider various tools with overlapping feature sets in case one tool misses an issue.

What tool can you use to detect major errors in your site and correct them to improve search engine crawling?
If you implement ARIA in your application, what effect would it have on your search engine ranking?
How can you check which parts of the site are slowing down when downloading?