If your looking for a folding bike to be as fast as your full size bike that's probably not going to happen. Folding bikes like the Brompton were not made for that reason. They were designed to be a bike to take along on a trip that requires transportation.
Folding bikes can be taken with you right into your apartment, onto elevators to your place of work.
There are now many good quality makes and models to choose from that alternately optimize ease of folding and folding size, performance, ride, and price.
Gears on small-wheeled folding bikes are or should be higher to compensate for speed but limited gearing on some models may slow you down. If that is a major concern for you, you might want to look more toward higher end "performance" folding bikes. If you plan to take a lot of trips that utilize transit, a bike that folds quickly and compactly may be best for you. If you are a tall rider, you may want to check out how each type of folding bike can adjust to fit you.
Bike Friday brand offers many types of folding bikes for those most concerned with performance. All Bike Fridays use 20" wheels and are built to fit into a suitcase for travel. Many also quick-fold and fit into a travel bag for use in commuting. A typical "stock" model quick-folds in 30 seconds to 27" X 27" X10" and weighs in at about 24 pounds. Bike Fridays range in price from about $730 for a stock model to about $3400 for their best custom-made bikes.