No, not really, post do not slow the site speed that much. Number of large images possibly, lots of post on front page with large images, yeah maybe if the images are 800x600.
W3 cache is good plugin, however there are other things that you could do to speed up your site such as:
- CSS Optimization and compression
- JS Library compression or minimization
What I do with every site that I make is this:
- One stylesheet - i.e. style.css (optimized)
- One JS file - i.e. scripts.js (minified)
- Valid Markup - i.e. W3C (http://validator.w3.org)
Keep everything as clean. You probably heared of this before:
Make everything as simple as possible, but not simpler. - Albert Einstein
Honestly I don't even use any plugins for cache and sites are running smooth and fast. Everyone can do the same thing.
Try this add-on for Firefox https://addons.mozilla.org/en-US/firefox/addon/231185/
I always recommend http://2010dev.wordpress.com to anyone that needs good/clean coded site, especially for people who just started using WordPress.
Remember: all this might not help much if your host is slow. I switched to HG 2 years ago and everything is blazing fast. Hosting is one of the reasons why some sites are slower in addition to all of the above.