If the logfile keeps growing that fast, you must have a lot of activity on the database. If it is growing to that size, it needs the space, you shouldn't be shrinking it. You're slowing down your database because it needs to keep growing the logfile. If your concern is that it is taking up too much diskspace, set the maximum size of the logfile to something that better fits your existing space and then look seriously at buying new, larger hard drives.

If you can, put your log file on a different drive from your database.