caching

tl;dr: Drupal 8's defaults make most Drupal sites perform faster than equivalent Drupal 7 sites, so be wary of benchmarks which tell you Drupal 7 is faster based solely on installation defaults or raw PHP execution speed. Architectural changes have made Drupal's codebase slightly slower in some ways, but the same changes make the overall experience of using Drupal and browsing a Drupal 8 site much faster.

When some people see reports of Drupal 8 being 'dramatically' slower than Drupal 7, they wonder why, and they also use this performance change as ammunition against some of the major architectural changes that were made during Drupal 8's development cycle.

First, I wanted to give some more concrete data behind why Drupal 8 is slower (specifically, what kinds of things does Drupal 8 do that make it take longer per request than Drupal 7 on an otherwise-identical system), and also why this might or might not make any difference in your choice to upgrade to Drupal 8 sooner rather than later.

I'm a huge fan of Boost for Drupal; the module generates static HTML pages for nodes and other pages on your Drupal site so Apache can serve anonymous visitors the static pages without touching PHP or Drupal, thus allowing a normal web server (especially on cheaper shared hosting) to serve thousands instead of tens of visitors per second (or worse!).

For Drupal 7, though, Boost was rewritten and substantially simplified. This was great in that it made Boost more stable, faster, and easier to configure, but it also meant that the integrated cache expiration functionality was dumbed down and didn't really exist at all for a long time. I wrote the Boost Expire module to make it easy for sites using Boost to have the static HTML cache cleared when someone created, updated, or deleted a node or comment, among other things.

I just discovered (after asking about it in the #drupal IRC channel) the wonderful little program ab, included in an Apache installation. This little nugget does one thing, and does it well: It beats the heck out of your server, then tells you how your server did in terms of page serving. I tested a few different configurations on a dedicated, 4-core, 4 GB RAM server from SoftLayer, and used the following two commands:

1. Download the specified URL 1,000 times, with KeepAlive turned off (each request gets a new http connection):

ab -n 1000 -c 5 http://ip.address.of.site/path-to-page.php

2. Downlaod the specified URL 1,000 times, with KeepAlive turned on (thus allowing the connection to be maintained for as many http downloads as you have set in your httpd.conf file):

ab -n 1000 -kc 5 http://ip.address.of.site/path-to-page.php

I ran these tests a few different ways, and here are the results of the tests I ran with KeepAlive on, with the number of pages per second ab reported listed after the method:

• Drupal - normal page caching turned on, css/js aggregation, 55kb page – 12.5 pages/sec
• Joomla - no page caching (disabled due to buggy 1.x caching), 65kb page – 8.2 pages/sec
• Drupal - boost module enabled, serving up the boost-cached file – 3,250 pages/sec
• Joomla - custom page caching system enabled, serving static html file – 2,600 pages/sec

Speed boost due to caching: ~250x faster!