An English source that *has* to advertise the "mighty longbow" in an article that ain't about it :p
Notice that it constantly uses the phrase "this researcher" and yet presumes longbow awesomeness at the beginning. How hard were they trying? And all that business about projectiles "flying" without saying anything about fletching? Jeez. Not to mention it's so cheekily written...
The original source where those measurements of time comes from I believe is an older book, "The book of the Crossbow" mentions additional info.
1. A trained soldier would load it much faster than an academic ("this researcher").
2. A great deal of that time is aiming NOT loading the weapon.
3. A regular bowman would never shoot at max speed (part of it is same reason that soldiers now don't spray and pray).
Meaning of course that rates of fire are much closer than what is generally perceived. The source isn't lying exactly but being extremely deceptive.
While dying horribly is a major part of DF, combat isn't the focus as much as building things IMO. Such changes could compromise the building of things. For instance you say to make the steel crossbows 2-3 times slower. Isn't steel 133% of damage? Doesn't that kill their dps? Such a thing would make the production of higher weapons pointless and make the management of resources easier as that's one less steel item you have to make. That would compromise part of the rising difficulty as you go deeper into the mountatin. The better solution, IMO, would be keep the steel xbow ROF the same but increase the amount of ingredients: loading mechanism, stock, etc.
Another game example is a castle builder, Stronghold. Crossbows there while slower they're only slightly so. The accuracy and power makes them across the board, with the exception of firing flaming arrows to light pitch, stronger than the bowman. However, they're still balanced combat wise because they take more materials to make, more gold, and require leather armor which needs cows. If you butcher too many cows you kill your cheese production and ruin food variety affecting happiness. The key is that the cost/benefit is focused on management not tactical hoohaw from the interwebs' idea of "realism." Stronghold 2 compromised this balanced by having bowman better. Combined with their ROF they took over every other unit on the field meaning you never had to build advanced buildings and you could rush straight out the door. It killed the game.