I have a very simple doubt relate to blogs on subdirectories. I have a main website built with WordPress and I want a blog with it.

When we read about the recommendation to put a blog on the subdirectory, like www.mydomain.com/blog, does it means I have to make a new WordPress install on a subdirectory or I just have to create a page called Blog and use it from the same WordPress installation of the main Website?

What concerns me is that my main website uses a very resource-intensive plugin (wpgeodirectory) and I'm afraid that a blog on the same install may affect the performance of its service.

I'm worried about SEO gain versus performance. The blog will have simple texts and images, nothing very different than the usual and it will be used to attract site traffic to the main website. Should I be worried about sharing the same WP install between the main website and his blog?