Hi Alessandro,
That is absolutely not normal... Understatement of the year, I know, but when you view the review request page, we're primarily working with the database and memcached and using the contents to render the page. Anything truly expensive (like adding in rendered fragments of diffs) happens after the page is sent to the browser.
The exceptions to that are:
1) If you use any custom extensions, that can always introduce an issue.
2) If it's Perforce, we may need to query the repository for a little bit of state (but we cache this in memcached).
To start off, a couple questions:
1) What version of Review Board is this?
2) Any custom extensions?
3) What type of repository are you using on this review request?
4) Can you verify that memcached is working and responsive (Admin UI -> Server Cache)
Christian