[The information in this post has been greatly updated - basically, Drupal 6 will enjoy even more of a performance advantage over Drupal 5 than is indicated here]
The more one learns about Drupal 6 the more there is to like. A while ago, right before Drupal 5 was released, I heard someone reference it being a 'developer's release' (think maybe it was Dries). That seems like an apt way to describe Drupal 6 these days. If the anti-CMS, ya-gotta-roll-your-own-or-it-stinks critics have a last breath left, this might be what snuffs it out once and for all. The list of new features, optimizations, and/or technologies introduced in Drupal 6 seems destined to light up the antennae of developers everywhere.
Performance, performance, performance
If there is any doubt about whether there will be performance improvements for Drupal 6, wonder no longer. Benchmarks, made using these standards, shows that the current, pre-beta, version of Drupal 6 performs 19.5% faster than Drupal 5.2 does when NO page caching is on. A healthy improvement by any standard, but even more so when one considers the following:
Drupal page caching is never active for logged in users, so any gains to non-cached speeds are pure gains for everyone that is logged in. This is a big deal when you consider that up till now it has taken 10x longer to serve authenticated/logged users than it does to server anonymous site visitors.
With the block cache patch which looks destined for core applied, Drupal 6 becomes 32% faster than Drupal 5.2 for authenticated users. Community site system admins rejoice.
Without quibbling over the details of specific numbers and/or benchmarking methods - one thing is very clear.
Speed is coming to Drupal.
Download benchmarks for Drupal 6, which includes benchmarks for block caching here.