We recently conducted a performance assessment for a client, and the main problem was something really simple, but also very detrimental to the site.
The site used the Service Links module, version 6.x-2.x.
The following are the performance figures for the site, in seconds, before any tuning.

3.67 http://example.com/product/215
2.64 http://example.com/product/572
68.32 http://example.com/list1
65.11 http://example.com/list2

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By design, the Drupal CAPTCHA module disables page caching for pages it is enabled on.
So if you enable CAPTCHA for user login and/or registration forms, those pages will not be cached. This is often acceptable.
However, if you enable CAPTCHA for comments, and have the comment form visible at the bottom of each node, then a big portion of your site's pages will not be cached in the page cache at all.

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With the "Cloud" being in vogue currently, we see a lot of clients asking for cloud solutions, mostly Amazon AWS. Sadly, this is normally done without really doing a proper evaluation into whether the cost is reasonable, or the technology is suitable for their specific needs.
Amazon AWS provides some unique and compelling features.

The session that I proposed for DrupalCon San Francisco 2010 on 2.4 million page views per day, 60 million pages per month, one server was accepted, and I just finished giving it.
Note that from the time we proposed the talk, until we actually delivered it, the site hit new records (2.8 million page views per day, 70 million page views per month).
Here are the slides from the session, which I designed to be useful on their own, even for those who did not attend.

For high traffic Drupal sites, using the memcache module is a common method for making sites scale. The memcache module uses the memcached object caching daemon for doing the actual caching.
In this article, we document how we ran into a limitation of memcached, and how we overcame it by a simple patch.

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