If you have a large forum with many users and a large database, executing the cron may indeed result in high load on a shared server. Reducing the size of the database should help significantly, but in most cases this is not really an option.
If the problem is caused by your cron job being executed too often, you can simply increase the interval for its execution. In any case, your host should have provided possible solutions to the issue and you may want to check them. If you do have a big and popular forum, maybe you should consider upgrading to a more powerful hosting solution.