I hope this is helpful feedback...
I was tracing the causes of a slow-loading site. I found that Contact Form 7 version 3.0 was taking up half of all plugin-loading time, via its initialisation of its own plugins, on all pages.
I worked around this performance penalty by deleting some of the CF7 plugins (though they'll come back when I next have to upgrade); but in future on new sites I'll work around this by using a different plugin, because the performance penalty is too high, especially as CF7 seemed to initialise on every page even if there were no contact forms on that page.
I used the plugin "SJ Hook Profile" to get the timings to find out which plugins were running slowly.