I was performing JMeter load tests on an installation of Drupal that had global redirect enabled. The test in question is a slightly modified version of the JMX found here (the only major modification being that I'm not testing the search functionality).

The sampling of URLs that I used with the test (which came from our access logs) included a large number of node/### paths.

What I found was that under loads >= 128 concurrent users, approximately 50% of all page requests returned "503 service unavailable". Of those that failed, almost all of the form "node/###" had a 100% miss rate (while others were more varied).

When I re-ran the test with all aliased URLs (and no node/### paths) , I was able to go even as high as 1024 concurrent users without much issue. Obviously that number will fluctuate based on your hardware, but it's an order of magnitude greater.

It seems to me that the extra path lookup and redirect taking place in Global Redirect (and potentially other factors) are creating a severe performance bottleneck.