At the moment, the Statistics graph on the home page is not optimized. Given the nature of the data that it's trying to report, if you get a site that has more than 500,000 rows in that page statistics table (which is not hard to do) computing that chart gets slow, and really gobbles up as much RAM (for its temporary tables) as possible.
Eventually, when we bring the statistics in concrete5 up to par with many of the other systems, this won't be an issue (we'll run a job to summarize these tables so performance is not an issue.) In the meantime, if you're using something like Google Analytics or Mint or something similar, you may find concrete5's statistics overhead unnecessary.
You can disable it with this command, in config/site.php