Once again, your usage will determine it. Ideally, a 1.8 GHz processor is one that is considered to be an average one. It will take 555 picoseconds, which is about half of one nanosecond, to do a job.
Light will travel 6 inches within that time. Considering this fact, a 1.8 GHz processor is quite fast, but middling fast. Most of the processors now runs at a speed of 3 GHz while there may be a few that may even run at a lighting speed of 5 GHz.