I personally built up my own little skill gym with my speedball, punchingbag and freestanding man, but to improve your speed with a heavy bag, well.

Heres to build up coordination, this is what i did in my earlier days :

Get a marker ( that can be washed off ) and draw sqaures and in them just have a letter like A, B, C ect.

Then you will have 1 minute rounds, in which rounds you will only use one type of punch.

then if you have a friend at with you, get them to shout out one of the letter and you must hit it as fast as possible and over the rounds get faster and faster.

but if you trully want to improve speed and coordination, buy a freestanding man, they are great for building up both.