With Sustained Fire, your firing rate increases by +25%/second, capped at 75% - so you need 3 seconds of staying still to reach the max. firing rate.
With Improved Sustained Fire (introduced in 1.15), the firing rate increases by +31.25%/second, capped at 125% - will reach the cap in 4 seconds.
See Gun Speed (shots/second) and Speed Perks for how it stacks with other Speed perks
