Yes, overlockers sew fast, usually 1500 stitches per minute and with a blade in action alongside the loopers so there is a lot happening. Keep all pins well away from the seam edge, contrary to normal sewing machine techniques, if you pin two layers together, then place the pins parallel to the fabric cut edge at least 2 inches away from the edge so the pin goes nowhere near the foot or blade.
Check whenever you use it, that you firstly extend the thread aerial up to full height, you need to ensure the threads can flow smoothly to the tension dials as the thread will stream off fast, even the slightest catch of a thread will snap one, throw your tension out or damage a looper.
As you've had it all this time and not used it, why not consider investing a few dollars more and go to a class on overlockers, and learn how to thread it up for three, four, rolled hem and using the differential feed. If there is no class options in your area, then try the Janome dealer near you and see if they would consider giving you a a little lesson on using it. You could also check out the videos on You-tube or read some pages from Debbie on www.sewing.about.com
, she has several pages on sergers (USA name for overlockers), explains the needles, threading, different techniques and maintenance. Or check out your local library for any books on sergers/overlockers, there are several good "how to" guides out there that would certainly help you get started.
Unfortunately there is no way to slow the speed to half speed, just practice with lots of fabric scraps at first, to get used to the sensitivity of the foot control. If you do dressmaking but aren't sure where to overlock or which seams you could only overlock, then consider just doing a three thread overlock around the edge of each fabric piece before you commence assembling the garment, just do your notch markings another way such as chalking them or using water soluable marker pen as the blade will cut away your outwards notches.
My other suggestion would be to make a simple sweat shirt on it using the 4 thread stitch and not touching your sewing machine at all. Once you've made one, you'll soon be wizzing them up in 30 mins, so quick to sew knits on an overlocker. Some of the commercial patterns are marked as suitable for sergers, look for one of these to try.
I hope this helps you to master your overlocker, a lesson or class would help you with the steep learning curve at first.