Plotted some data and found out that the brew does increase the final action speed (base + skill + forge) by 2x.
Interesting things to note:
- Gaps where increased action speed doesn’t do anything, then jumps considerably
- Mega fast brew doesn’t do much on lower action speeds, 7% increase with action speed 5, 100% at 40, 250% at 70
Confirmed the forge behaviour with just 2 samples, I’m not going to forge speed on all tools just to get more samples 
edit: fixed a few things in the chart, now says dex instead of agi…