I'm pretty certain that few people have ever seen a speedkill Party in action, even if you have heard the term applied.
Speedkill is a Partying strategy designed around a completely different fighting strategy than a normal XP PT. The standard XP PT setup is a heavily defensive setup: one single character (the Tank) is massively optimised to take hold and survive the hate from the target, and other Party members keep the Tank in the fight and with the hate. XP PTs prioritise hate control and survival over raw damage output.
So how does Speedkill differ? Well, for a start, throw out your notions of hate control and single tanks. The strategy for a Speedkill PT is to engage the target and cause maximum damage output in as short a space or time as possible, and then to move on to the next victim as quickly as possible. Hate can be traded between multiple PT members in melee, as it is almost impossible to assure 100% hate control in Speedkill.
The strategy of speedkill can be summed up as: Kill them before they kill us.
Jobs and their role in Speedkill:
MNK: One of the king jobs of Speedkill PTs, MNKs do high consistent damage, have the HP to survive drawing the hate, have abilities that complement occasional hate gain (Counter, Guard), and have a succession of deadly weaponskills. Chakra also aids them in negating downtime.
SAM: High TP gain, Meditate for instant TP return and Third Eye for survival when they take the hate attune SAM very well for Speedkill play. A selection of viable subjobs allow them to fill in a variety of roles at some levels.
WAR: Another job that can dish it out and take the punches when they come, WAR excel in speedkill. Subjob selection changes their individual tactics, but not their suitability for the task.
RNG: Their high damage output helps in speedkill, but RNGs tend towards a certain fragility compared to MNK, SAM & WAR. While Utsusemi is a great survival tool, it also slows down their damage output - and in a speedkill party where hate control is not a priority, they may spend more time than usual casting.
PLD: The PLD job is optimised towards Defensive prowess, and is not suited to the tactics of speedkill. Their reliance on MP also counts against them, as they require downtime that would be better avoided.
DRK: This job has a marked tendency towards high "spikes" of damage, using their attack boosting abilities to make a single massive damage assault, generally in an attempt to close a conventional fight. In speedkill Damage over time is emphasised more than Single Point Damage, which is not the DRKs forte. Their reliance on MP also hinders them in speedkill, unless fighting targets which can be Aspired for MP. Depending on your selected target, DRK may be viable for Speedkill.
DRG: DRGs are very useful in some Speedkill setups, mainly depending upon the selection of targets, and the selection of the DRGs subjob. DRG/WHM can be very effective is minimising downtime, with healing (and status healing) breaths from the wyvern. DRG weaponskills cause high damage and Lances create good damage generally. In addition, DRGs have the option to simply drop the hate if they take too much damage, via their Superjump ability.
NIN: With their ability to survive hate based on Utsusemi, NIN can work well in a speedkill party, with some possible alteration of equipment for the circumstances. NIN Dual Wield abilities give them a decently high DoT and a high enough TP gain to unleah good weaponskills. They also have the ability to unleash some high damage ranged attacks.
BRD: Probably the first choice of support class for Speedkill, BRD buffs require no MP, thus creating no downtime, consistently regenerate the entire Party and return MP to mages, increase the lethality of the Party and decrease the targets resistance to that lethality, and provide supporting healing magic.
SMN: An excellent job for use in Speedkill, SMNs buffs are exceptionally useful for reducing damage taken, and are actually enhanced by the hate-trading typical of a Speedkill Party, as such buffs as Stoneskinga & Blinkga are applied to all party members, and used by all, thus multiplying the SMNs usefulness compared to the typical single tank PT setup. In addition SMNs can (and in speedkill should) do consistent DoT with an appropriate elemental avatar, and hate gained from this essentially does not exist as far as downtime contributions go. Their MP regen passives help in reducing downtime, especially if stacked with a Refresh or Ballad effect. Support healing is also a plus provided by a SMN/WHM
WHM: Damage is an inevitable part of a fight, and Speedkill partys, which concentrate on dishing out damage over surviving it, can take a great deal. WHMs are an obviously complementary part of the party setup, especially against undead where Holy can cause very good damage. However, because unlike a regular PT setup hate is shared among the PT members, there are effectively more HP and defences before any one member is in critical condition. WHMs are nice, but not as essential as for normal partys, as the use of Curaga spells by multiple WHM subjob players can be as effective.
BLM: Speedkill tactics create a lot of opportunity for Magic Bursts and general nuking, but the less rigorous hate-keeping standards of a Speedkill party can cause issues when faced with the sheer destructive power a BLM is capable of. In addition creating Magic Bursts for Ancient Magic generally takes longer than a Speedkill party wants to spend on a fight. More than even RNGs, BLMs face the distinct possibility of killing themselves by excess hate in Speedkill. Spells such as Shock Spikes (to Stun attackers), Stun, Blink, Stoneskin (from WHM sub or SMN buffs). Once again, /WHM is useful for downtime minimisation. Aspir makes their downtime minimal against the right enemies.
RDM: With the ability to effective negate downtime with their Convert and Refresh abilities, provide some nuke damage support, good healing cover, dispel enemy buffs and take hits in the event that they draw hate (quite likely in Speedkill), RDMs make a very good choice for a Speedkill party. Depending upon subjob they can Aspir as well to keep MP at top levels, or they might be able to supplement their healing with status cures and Curagas.
THF: The main thing THFs bring to a conventional party is their supreme hate control abilities, which find less of a place in Speedkill as the placement of the mob upon a single tank is actually contrary to the prefered method of fighting - that being to trade hate to share damage among several HP pools. Depending upon subjob their DoT can remain high enough to compete. In the event that they draw hate they have the excellent passive evasion bonus abilities.
BST: Although uncommonly seen in partys at all, BSTs can do very well (even in conventional partys). One of their major advantages is something they share with SMN & DRG - that their hate is essentially divided between them and their pets. BSTs can also do very good duty at using their Charm and Calm abilities to control dangerous situations. Physically, BSTs are no slouch in terms of HP, Defence and damage output themselves.
Speedkill tactics can be summed up as fast and furious. Weaponskills and job abilities fly like fury, and skillchain effects hammer into the target often. In Speedkill it is often necessary to keep the party moving rather than camping in one spot. There is frequently no officially designated puller, with the next target simply being whatever is available. The aim for a speedkill party is to kill quickly and move on.
There are various good targets for Speedkill. Choosing a target with elemental weakness to whatever very high damage weaponskills and skillchains you can create is important, as is selecting targets according to damage type. Some of you may have seen one of the videos of a party of MNKs fighting bones and using Fusion reaching absurdly high XP chains, a classic example of Speedkill fighting.
Conclusion: Speedkill naturally favours jobs that have a balance, rather than a specialisation, in combat. Melee jobs that excel are those with both the ability to take and to receive damage. Mage jobs that can do multiple duties well shine in a Speedkill scenario.
Speedkill is outside the box for the way you may have learnt to fight in this game, but it does work, and work very well, especially for those jobs that have the required balance of traits and abilites.