XML as buffer between server and mySQL?

Hello,
I’m redoing a site. The main focus is optimizing speed and stability.
Right now the site is slow, it’s faster than it used to be because recently XML was introduced in the structure.

users don’t interact directly with the database but instead all their actions are stored in XML files, and every 30 mins the XML files are read by the DB and the DB updates it self.

Should I implement the new version of the site the same way.
Do you think that what was done with XML really did help the site, or do you think the server was just acting bad the previous days.

Any other tips on speeding up mySQL queries?
Thx