First of all, I will make sections for those people who go by the rule TL/DR
I am a fan of Dual Zorens since... since very long time...
In the past, I made comparison between melee weapons with high speed and high crit. Recently, I've noticed how great Naramon Focus tree is for using them (mostly for Shadow Step).
I wasn't really giving it much thought, I just went with anything that has even a "little bit of crit". But then I got interested in Excalibur's Exalted Blade. What weapon with which mods would be the best etc.
But the final push for me was (as usual) comment section below Dual Raza article. So I thought "I think it's time for another Fast'Crit weapon comparison".
The results are quite interesting (for me at least).
|Unmod AvDMG||Unmod DPS||Mod AvDMG||Mod DPS||MaxSpeed||"+Zerker"|
|Dual Kamas Prime||80,5||94,185||319,858||767,179413||1,521||2,40|
|Prisma Dual Cleavers||52,5||52,5||252,742||518,1211||1,3||2,05|
Sorted by Master Rank requirement (0,0,2,3,3,6,6)
So, let me share some insights about this comparison.
DUAL KAMAS - For this one, I felt like I was judging the fish by it's ability to climb a tree. This is indeed a fast weapon, but it ain't crit one. I placed it here mainly to show the difference between Kamas and Kamas Prime, but also Dual Raza, which requires Dual Kamas to craft it. Overall, worst weapon on the list. It is much better to avoid it, unless you want to level up Mastery Rank / build Dual Raza. The good thing about this one is that you don't have to bother with crit chance and damage mods and maybe put 2 elemental mods instead.
DUAL CLEAVERS - Just like before. This weapon is still worse than Dual Ichors and Dual Zoren, even with Justice Blades mod. It's like Dual Kamas, but it has higher crit but lower speed. To make it even worse, if you want to put Justice Blades in it, you will have to sacrifice one of the core mods for a melee weapon.
DUAL ZOREN - Without the legendary ability called Zorencopter, this is just plain Fast'Crit weapon. It is still pretty decent, especially for a weapon you can craft early on (MR:2) and mods you put on it scale well with Exalted Blade. This is the easiest to get Fast'Crit weapon.
DUAL ICHOR - Now this one is different. You can mod it to deal 2 types of mixed elemental damage and with that attack speed, you can expect often status procs, even when it has base 15% status chance. Bad side of this weapon, IMHO, is that is purely situational. You can use Corrosive+Heat for most enemies, but for best results, apply elemental mods according to the challenge.
DUAL RAZA - The main fuel to the fire for this comparison. Is it better than Dual Kamas? Yes, definitely, but it is much, much slower. Second slowest weapon on the list (As a part of Soma gear, this kind of also has it's own spin-up time with Berserker mod lol). Also another notable difference is that it requires MR:6 to craft. Dual Ichor is a better weapon here. Additionaly, it has total of 32% crit chance with True Steel, 8% less than best crit chance (with this one mod), but you can expect it to crit hard once every 3 swings.
PRISMA DUAL CLEAVERS - As a weapon from the Salt Trader, I expected an upgrade from Dual Cleavers and there it is. Second best Fast'Crit weapon on the list. Bigger slash Dmg (Jagged Edge scales a little better) and much faster attack speed than normal Cleavers. It feels like it was ripped off some random neon sign from New Vegas and shaped into a weapon of justice. Speaking about justice, it can also use Justice Blades mod, which doesn't really increase damage that much (only +35) but it causes Justice proc, which after collecting merely 2k affinity, triggers 1000 Blast Damage to enemies in ~20m radius, restores 25% of player's health and very temporarily increases armor by 25%.However, just like with normal Cleavers, you have to replace one of the core mods for the weapon (I recommend Fury or one of the elemental mods). Also, +35 Dmg might not seem much, but it means extra 200 crit damage. If I would've been comparing Utility instead of just pure damage, this would be no. 1 on the list, but in this one, the best weapon is...
DUAL KAMAS PRIME - The newest addition to Fast'Crit weapons and they are nothing like their non-prime counterpart. They are strong, deadly, and fast. When Berserker mod winds up to the maximum, you Warframe will transform into a space lawn mower. You might even build it for double elemental combo and it will still work fine, unless you want that crit chance and damage for Exalted Blade. Only the big drawback is that this weapon has (beside Dual Kamas which isn't crit weapon at all) lowest crit chance and crit damage of all Fast'Crit weapons. The second bad thing is that it is currently in Prime Access, so (according to biased lower drop rates of prime access items) you will have a bad time farming for this and the market price is really high (~200p, twice as much as Prisma Dual Cleavers, which you can only obtain through trading).
So, I came, I saw, I made a mess. I might have said something wrong or I might be just not informed enough - please, feel free to write your comments - I will definitely read them all (and if neccesarry - make adjustments)
TL/DR - Prisma Dual Cleavers and Dual Kamas Prime are the best Fast'Crit weapons at the moment.
Thank you very much for reading, have a good day.
Good Luck and Have Fun!
 - Core Mods: Pressure Point, Jagged Edge, True Steel, Organ Shatter, Fury, Berserker, Elemental Mod 1, Elemental Mod 2 (disclaimer: some of the mods posted might only apply to weapons stated in the blog)
 - Zorencoptering - an ability to use Midair Spinning Slide Attack in order to propel your Warframe to an insane speed in a blink of an eye. In the past, it was used by players to traverse the area with great speed, depending on weapons and mods used. (LINK)
@edit: This was a very nice experiment for me, but I am starting to see a few flaws in it. Comments made me realise that I used too wide spectrum of weapons and used mods I shouldn't have. I will make an improved version of this once I have time and more information. Meanwhile, please keep posting constructive comments - they will be a big help.