It is normal for an engine to be hard to turn by hand with the plugs in, you probably also hear a hissing sound when you try this. the reason this happens is because of compression. A engine is just an air pump and if the air has nowhere to go it creates back preasure. Now for your slow spinning starter. I would be willing to bet you do not have a ground from your engine (not your battery), but your engine block or heads to the frame. This will cause the problem you are experiencing. Most production cars have a small ground strap from the back of the heads or manifold to the firewall. I prefer to run a quality ground from the block to the frame. So before you tear anything apart I would make sure you have a nice healthy cable or strap (not 8, 10 , or 12ga wire). Also make sure both attaching points are clean and it is a good idea to use a star washer as well (a little conductive grease wouldn't hurt either).