There is a very nice plugin called P3 (Plugin Performance Profiler). Give it a try!
This plugin creates a profile of your WordPress site’s plugins’
performance by measuring their impact on your site’s load time. Often
times, WordPress sites load slowly because of poorly configured
plugins or because there are so many of them. By using the P3 plugin,
you can narrow down anything causing slowness on your site.
- This plugin uses the canvas element for drawing charts and requires
requires Firefox, Chrome, Opera, Safari, or IE9 or later. This plugin
will not work in IE8 or lower.
For your second question you could read WordPress Benchmark: 3.0 vs 3.1 vs 3.2 vs 3.3.
text only mirror in google cache
WordPress now has 4 versions in 3.x line. With slow adoption rate for
previous two major versions, despite great 3.2 release, question is
will the new WordPress 3.3 manage to persuade users to upgrade? This
benchmark will try to give, at least, partial answer to that.