Click my username and then click through to my most recent post and scroll down to the end of it, so you know what I mean in the rest of my comment. Know that I have 970 published posts. My blog is set for infinite scrolling and I have a strong traffic flow. However, it loads faster than 88% of all tested websites.
Actually most of us reduce page loading time by not displaying full posts on the front pages of our blogs. These days visitors are using a variety of devices to load our blogs. Some are still on dial-up and paying for connect time and we have 3 - 4 seconds for the front page to load before they start clicking out.
The setting here > Settings > Reading is only for the RSS Feed summary and does not affect the posts. Setting it up drives traffic to the post rather than reading it in email or the Reader so we get visits to the full posts that create page views on the blog.
If you want to display only excerpts on the front page of the blog followed by read more links, you can insert "the more tag" into each post prior to publication.
Alternatively you can switch to using a theme that automatically displays front page excerpts. This is a list of active themes on WordPress.com that use the excerpt instead of full post content. Here are the themes that do provide that feature > http://en.forums.wordpress.com/topic/theme-with-automatic-excerpts?replies=13#post-1396410