I'm wondering if the speed weapon special ability would grant a +1 to the critical threat range to a Focused attack. You could take the Improved critical feat to also get the double critical threat range.
On a Fighter you would end with 11–20 critical threat range (18–20 for a scimitar, 15–20 for improved critical feat, 11–20 for giving up four attacks with Focused Attack and a speed scimitar with the improved critical feat.
Is it even worth to give up all attacks to get a super good critical threat range? Or is it broken and we shouldn't use this 3rd party feat?
Worth it? No.
If your main hit crits when you roll an 11, then the 4 attacks you gave up would have probably hit at least twice. If your crit only gets x2, you loose out. If x3, then you match. But wait... those extra hits may also crit. So at x3, you still loose. If you have a x4, it might be worth it. Otherwise, no.