Gotta make a claw grip and glide the knife behind your knuckles, you don't have to go fast but a knife is definitely faster than the plastic chopper or food processor because cleaning takes so much time. You need a SHARP knife as well, almost any knife from a store will not be sharp enough. I prefer a Gyuto which is an in-between knife of a Santoku and Chef Knife.
Also cut on a wood cutting board is ideal or polypropylene. If using a wood cutting board it should be maple, companies sell all types of cuttings boards made with bamboo,teak, acai, etc. All those woods will dull your knife SUPER quick, its just as bad as cutting on glass or stone/ceramic, most folks don't really have sharp knives so the boards don't matter but when you spend time sharpening your blade you don't want to fck it up.
Also your grip should be like a pistol grip with your index finger under the back of the blade pinching it near the beginning of the handle. People cut with their index finger on the the spine of the blade and that's a recipe to cut yourself as well ad put more effort than necessary into chopping, if your knife is sharp you should be able to pull it back like a saw blade with one stroke and cut your food, or rock the blade up and down like a see-saw to chop while keeping it down low on the cutting surface.