The Boost is often a great help with speeding up web sites of small to medium size and/or hosted on shared hosts.
It works by writing the entire cached page to a disk file, and serving it entirely from the web server, bypassing PHP and MySQL entirely.
This works well in most cases, but we have observed a few cases where boost itself becomes a bottleneck.
One example was when 2bits.com were called to investigate and solve a problem for a Fortune 500 company's Drupal web site.
The site was configured to run on 12 web servers, each being a virtual instance on VMWare, but all of them sharing a single RAID-5 pool for disk storage.
The main problem was when someone posts a comment: the site took up to 20 seconds to respond, and all the web instances were effectively hung.
We investigated and found out that what happens is that boost's expiry logic kicked in, and tries to delete cached HTML intelligently for the node, the front page, ...etc. All this while the site is busy serving pages from the same disk from boost's cache, as well as other static files.
This disk contention from deleting files caused the bottleneck observed.
By disabling boost, and using memcache instead, we were able to bring down the time from 20 seconds to just 8 seconds.
Further improvement could be achieved by using Varnish as the front tier for caching, reducing contention.