Splitting One Big WordPress Site into Multiple Small Sites

I have a very big WordPress site and I am facing speed issue and server resources issues. Specially numbers of MySQL records are so high. So, I want to split the big WordPress site into multiple small sites.

I have URL structure like this

1. http://siteurl.com/this-is-first-post.html (category1)
2. http://siteurl.com/this-is-second-post.html (category2)
3. http://siteurl.com/this-is-third-post.html (category1 and category2)

I want to separate the WordPress according to category, I am thinking about two solutions

1. Either I create two DBs for each categories, e.g. db_cat1 and db_cat2 and I divide the data using some logic.

2. Or I create subdomains and install different WP instances like this:

   • http://category1.siteurl.com/this-is-first-post.html
   • http://category2.siteurl.com/this-is-second-post.html

My questions are:

1. If I try the first logic, then how can I divide the databases into different categories db?
2. If I try second logic, then how can I manage the URL structure and keep SEO juice there? How can I divide the common tags and categories?
3. How I will manage resources such as image etc after splitting the WordPress?
4. Is there any better way to do this job?

I don’t want change the server or work on server because I already tried different types of server and all servers have the MySQL resources issues. I prefer second logic because it is easy to handle.