Slow site due to link rel in header

So i’ve been battling with a speed problem on my WP site for 3 months now, and I finally found what I believe is the issue. First things first, about the site;

2.7.1 / 15,000+ Posts / 29 Categories / Decayed theme
Dedicated Dual Xeon 2.4Ghz / 2GB RAM
2 Other sites on the box have pages loading ~1 second
WP site loads pages at ~10 seconds (Google Webmaster Report)

I tried debugging all plugins, optimising, caching, the works, but nothing was solving it. Then I found a site that reports the speed of each individual item in your page (Octgate Site Timer).

Here’s my results.

The link rel data in my header is taking 30+ seconds to load(!), especially the archive pages. I have no idea, but a quick test proves that’s the case. So I went into header.php and removed this line;

<?php wp_get_archives('type=monthly&format=link'); ?>

I cleared all the cache, and ran the test again… yup, it loads in a third the time. The remaining delay is the link rel for feeds.

Why would that code being slowing my site down so much? Is it because it’s got a lot of posts? (1,000+ per month).

I’m happy it’s solved, but would like to understand what’s going on, and document the issue here for other users.

Thanks,
n