I was putting together a document for my clients on how various hosts perform and the results were in some ways surprising but in others not (image in the follow up). This entire set of tests is based on a modified benchmark script that I've mentioned around here before. What is important is that the tests focus on specific aspects of server performance, non-inclusive of actual server connection speed. They spit out results for PHP speed, image speed, and raw HTML speed.
A brief summary of my findings:
* A WiredTree VPS is faster and more flexible than an WP Engine shared account for a third of the cost.
* WP Engine is the the fastest of the shared hosts but has only average PHP script performance.
* InMotion is a great host if you use a cache/cdn solution to serve your static files.
* MediaTemple will see the biggest boost from a CDN since its HTML and image speed is awful.
* GoDaddy has gotten better in the last year, against all odds. Their slow image speed means if you serve static assets from a CDN you will see them move from annoyingly sluggish to slightly above average.
* Avoid DreamHost like the plague. That's a VPS test. The shared server numbers are far worse.
* AECNU's wpmu-hosting.org platform was repeatedly un-testable. I haven't delved into why it chokes on the benchmark script. Casual observation indicates above average static file speed and below average PHP speed.
Comparative image up next --