Here is a table that explains how much ias is needed to hit the desired fps.
N()()B TIP: FPS=frame rate per second(the more fps the better, but downfall is giving up other things for the needed ias)(7fps is considered double shot which means you tend to see people shoot 2 times for every shot they take)
(table may very depending apon the base ias of the weapon you are using, just use your math skills
CROSSBOWS:
Weapon Speed Breaks - Base:19 WSM:0IAS EIAS Frames
0 0 19
7 6 18
14 12 17
22 18 16
32 25 15
48 34 14
68 43 13
99 54 12
152 67 11
BOWS:
Weapon Speed Breaks - Base:13 WSM:0IAS EIAS Frames
0 0 13
9 8 12
20 17 11
37 28 10
63 41 9
105 56 8
200 75 7
It is not possible to get lower then 11 fps with ANY CROSSBOW, even if you have a 6 socket colosus crossbow socketed all with shaeles, that is the barier for crossbows. Bows their may very well be a 6 fps, but it is not possible unless you have a 200% ias ith bow which i think nobody has made yet