1) Open up TES Construction Set
2) Go to File > Data Files and select Morrowind.esm (expansions too if you want to edito something that comes with them), click OK, wait till everything loads
3) Go to Weapons tab
4) click on Type co types get sorted alphabetically
5) Find your crossbow (marksman type)
6) Double clock on it
7) Edit the "speed" value - lover it for a slower reloading, raise for fast shooting
8) Click save in the weapon's properties window, repeat for other crossbows
9) Go to File > Save and type your plugin's name. Save
10) Open up Morrowind Launcher and check you newly created plugin
11) Enjoy