I’ve been maining Slayer for a few months now, mostly as a league starter uber lab runner. I’m not a super hardcore player, but I like to try to start running uber labs within about 24 hours of the start of the league. I think that Slayer is by far the best way to do that because Slayer leech is completely OP in the lab for traps and the spikey damage of izaro and argus, but the way it works is really complicated. I didn’t really understand it myself until I read up about all of the mechanics and built a spreadsheet to explain it. Now that the rest of you are going to be leeching over time like a noob along with me next patch, I figured I would share my insights into Slayer leech since I’ve noticed a lot of people on this sub don’t really understand the weird leech mechanics. Sorry for the wall of text followed by a spreadsheet, but it really takes that to understand all of it IMO.
If you’re mapping as a slayer, you can probably assume you have your full max leech rate at all times. However, if you’re killing bosses (which is the only thing that ever kills my slayer) leech is much harder to keep maxed. That’s why the details matter.
So the way that leech works in Path of Exile is really complicated and unintuitive when it’s not instant. The 4 contributors to how much you leech are: how much damage you do, how much “damage leeched as life” you have, how much “increased life leeched per second” you have, and how much “% of maximum life per second to maximum leech rate”. They all do kinda weird things that aren’t evident from their wording, and you can read about that here: https://pathofexile.gamepedia.com/Leech
My TLDR of that page is this:
A single hit creates a single instance of leech that regenerates a base value of 2% of your life per second for a duration determined by how much total health the leech stack is worth.
How much damage you did in that single hit multiplied by your leech (“damage leeched as life”) is the ONLY factor at all in the DURATION of that leech instance. It has no effect whatsoever on how much life per second that leech hit gives you, ONLY how long it lasts.
If you have 1000 life, hit for 1000 damage, and have 100% leech, you will regenerate 2% of your life (20 hp) per second for 50 seconds.
If you want to regen more than 2% of your life per second for every hit you do to a mob, you need to get the “increased life leeched per second” stat. This value increases the regeneration of each leech stack by the percentage of the stat you have, but does NOT affect the duration. It makes you leech more health from the same damage you did. Large values of this (like in the Slayer talent Brutal Fervour) help a lot if you attack slowly or just don’t do much damage yet. Vaal pact doubles whatever your value of this stat is.
If you have 1000 life, hit for 1000 damage, have 100% leech, and have 100% increased life leeched per second, you will regenerate 4% of your life (40 hp) per second for 50 seconds.
Finally, these single leech instances stack until you have enough to reach your maximum leech rate. The base max leech rate is 20% of your total life, but this can be increased up to 29% on the tree and Vaal pact can double that to a maximum of 58%.
Slayer leech is special for two reasons. First, the leech stacks you gain when you do damage all go away when you reach full health without “Endless Hunger”. This allows a slayer to keep their huge regeneration from leech for a few seconds if a boss moves away. Endless hunger also gives you 20% leech on your overkill damage. 20% leech is a huge amount, and as we learned above that gives you an insanely long duration (but small magnitude) leech stack. Map monsters are easily overkilled for huge leech stacks, but if you’re going to be fighting a boss it can be a good idea to bring along a Writing Jar Flask which creates 2 worms with 1 HP on use that provide an insanely big overkill that will regeneration you for minutes.
The only way that you can leech more than your maximum leech rate is by having increased recovery of life, which is provided by a few sources, notably (Scion) Trickster and the Soul of Akaali Pantheon power. If you get 100% increased recovery you could get up to 116% of your life regenerated per second by leech. If you don’t know how Soul of Akaali works I explain it in the spreadsheet.
All of this is weird, and changing different stats does wierd things that you wouldn’t expect, so I made this spreadsheet to play around with the various values to get a feel for things:
The base values I entered there are a tooltip for my ice crash lab farmer as it looks early league when I start uber labbing, but you should play around with the damage, attack speed, and leech modifier numbers.
I tried to explain all of the calculations that I use in there, but the lessons it taught me are this:
Slow, hard hitting skills (like Ice Crash) take advantage of slayer leech better than fast, small hitting skills (Cyclone, blade flurry). If you overkill some worms from your worm flask with an Ice Crash, that leech will last over a minute with shitty gear. If you cyclone through them it will be closer to 10 seconds. The faster you attack, the less slayer overleech differs from regular leech. With early league gear, cycloning a boss with a two hander will give you leech that only lasts around 2 seconds, so overleech is barely noticeable. My ice crasher’s leech lasts for about 8 seconds on the first day of the league.
If you’re not attacking particularly quickly (aka not using cyclone), you get a huge benefit from increased life leeched per second even if you’re doing a shit ton of damage because it will take a long time to build up your leech stacks to max otherwise. Also, if you’re going to use flask worms for overkill to provide a baseline regeneration for yourself, you’re only going to get a measly 2% regeneration per second per worm unless you get some increased life leech per second. The main sources of this are the slayer talent, the vitality void cluster, the Carnage heart amulet, and Cherribrum’s body armor.
Ice crash (using Hrimsorrow gloves) works better for sustaining leech longer than any other skill I know of because it’s really easy to scale for big, slow hits. 208% base damage at level 20 is insane, but it also allows hatred aura to essentially be a 36% MORE damage multiplier since your damage is pure cold. Stacking cold pen essentially doubles the effect of hatred on bosses. Since ice crash benefits from elemental conversion scaling, you can have a 6 link of strong more multiplier gems that all increase your damage (Ice Crash, Melee Phys, WED, Ele Focus, Conc Effect, Cold pen) without having to use things like multistrike that increase your dps but lower your single hit damage. I would have to write another giant post to fully explain why ele conversion (especially to cold) is so good, so I’ll leave it there for now.
Finally, as a general Slayer tip, you can build a slayer almost completely generically for any 2 hander using almost any skill. I use this base tree every league with slight modifications for what weapon type I want to specialize in: www.poeurl.com/bDIy As long as you level up the different support gems, I can be running uber lab as full conversion ice crash and then change out my main skill links and my gloves and be mapping as full phys cyclone. It’s an insanely good league starter if you either don’t know what you’re going to way to play or have different skill/weapons in mind for different purposes.
TL;DR: Non-instant leech works weird. Read the wiki page on leech mechanics and play around with my spreadsheet (https://goo.gl/CTizD4) if you want to understand it better.
Edit: a lot of people have been asking about my ice crash build. I made a rough version of the my lab character for my own reference. Check him out in PoB or the PoE website. The character is AnubisMF and the account is MastermindPK. The tree is accurate but the gear isn’t balanced for resists and the gems besides those in the 6 link are wrong. Ill probably put a rough guide together at some point. Feel free to ask any other questions you have and I’ll try my best to answer!