Good pressure in water line. When faucet is installed pressure is about 20%. Screen filter is not the problem.

Your pressure should be the same as before. The difference you see is probably a lack of volume. Probably due to the little selector ball valve being incorrectly installed into the faucet. It's just not letting it turn on all the way. Since you have the same issue with the hot and the cold, the valve is the common denominator. If it were on or the other, I'd say check the lines or fittings.

Gdluk