Cron.php put the site very slow

Hi, I'm having a problem with my site. I created a crontab and make the admin/cron.php run every 30 minutes. But when I do this it always put my site very slow.

I know this file is required for some modules and blocks to work, is it really necessary?

Running the file manually I've seen that the task named "Running clean up task" take all the time. All other tasks run fast. Can I disable the clean up tasks?

Any advise?

Thanks