240 fps is optimal and the most accessible since you can lock it in-game.
you need to learn about key actuation and how it can impact animations, so i'll give you a brief overview:
on top of limiting the length of an animation with fps, the only way to tbf is by making sure your block is activated for as minimally as possible. however, the ability to keep a key activated minimally is heavily dependent on your hardware (your keyboard and/or mouse sensors.)
if you're using your keyboard to block (expensive):
preferably use a mechanical keyboard with low necessary force and one that uses repeated actuation. the only kind of mechanical keyboard i know that reliably works on gunz (and is by far the most expensive of any mechanical keyboard) is one with topre switches.
be wary that most, if not every, mechanical keyboards tout their nkey rollover (you can press as many keys as possible without overloading your keyboard sensor). most gunz moves require a keyboard that can repeatedly actuate (i.e. if you hold down 2 or more keys, it will spam those 2 keys repeatedly in a certain predictable order), and most mechanical keyboards aren't designed to do that inherently. in other words, don't confuse nkey rollover as something that will inherently guarantee success.
make sure you do your research before buying if you go this route.
if you're using your mouse to block (preferred: much cheaper and much easier to learn and improve upon compared to keyboard):
you need a mouse with omron switches. logitech uses the omron D2FC-F-7N microswitch in almost every mouse, but you need to make sure that the lip under the chassis is designed well enough to make the most use of it. by far the best mouse to use on gunz is the logitech g9x, but be wary that the mouse sensor has built-in positive acceleration.
also, make sure that you have all the extra lighting features disabled (light map, dynamic lighting, etc.) because it is notorious for causing microjitter.
once you get a hold on one of the aforementioned components, the best way to tap is to use the underside of your finger to force the key ever so slightly down. you can practice the minimum force necessary to actuate a key by locking yourself on a ledge (void slash off an edge slowly while lined up as straight as possible until you hover) and spamming slash-block while facing a wall. you should be "butterflying" seemingly as fast as a lawnmower if you're going to be able to tbf.
good luck! game's dying, so i don't recommend that you pick up what i suggest in the end because the skills needed to tbf with are not transferable to any other game.
(note that you may possibly be able to tbf with any mouse or keyboard, but you're not going to do it repeatedly and accurately without proper equipment.)