I just discovered that the Sustained Fire perks have been silently rebalanced (buffed and nerfed) recently (either in the 2.2 or 2.3 release; I suspect it was in 2.3 with the release of the Smart Minigun)
First, the buff; the rate at which the speed increases has been improved:
| Perk | Previous Speed Increase/s | New Speed Increase/s |
|---|---|---|
| Sustained Fire | 15.00% |
25.00% |
| Improved Sustained Fire | 25.00% |
31.25% |
Second, the nerf; the maximum speed increase has been reduced:
| Perk | Previous Max Speed Increase | New Max Speed Increase |
|---|---|---|
| Sustained Fire | 100% |
75% |
| Improved Sustained Fire | 150% |
125% |
Let’s look at the effective speed increase for different durations of standing still (italics is after reaching the maximum, bold is the higher value, some numbers rounded for clarity)
| Perk | 1s | 2s | 3s | 4s | 5s | 6s | 7s |
|---|---|---|---|---|---|---|---|
| Sustained Fire (Old) | 15% |
30% |
45% |
60% |
75% |
90% |
100% |
| Sustained Fire (New) | 25% |
50% |
75% |
75% |
75% |
75% |
75% |
| Improved Sustained Fire (Old) | 25% |
50% |
75% |
100% |
125% |
150% |
150% |
| Improved Sustained Fire (New) | ~31% |
~63% |
~94% |
125% |
125% |
125% |
125% |
So if you stand still for 5 seconds or less, the new version is an improvement - so for most runs this is a buff. Only in Endless/Endless Boss/Dark Castle Runs where you have God perks and can stand still for long periods is this a nerf