PoE 3.19 99% reduced projectile speed spell and attack demos
Timestamps below
rolling magma / eye of winter / possibly spark are probably the main viable spells. For attacks, helix / molten strike. I haven't tested spark collision, though (re-hittable every 0.6 second). For spark you need a really high rolled simplex amulet, though, since spark has 40% increased speed at lvl 20
Magma orb is usable but you have to keep speed somewhere between 80 and 90% or the speed / targeting becomes bugged (shown in video)
For the collision based spells like fireball, the hitbox is very small and only active for a few frames per second, basically whenever, they move a tiny bit, so you can't just stack up a bunch of projectiles in the air. I tried this in ledge, getting monsters to run into the fireballs, but it's difficult. Mark_GGG explained it here when they nerfed / fixed negative speed fireball. / h8mep0q
Veil or craft increased proj speed on 20% speed quality simplex amulet, and then mirror at lvl 8 difficulty for 2x mod, or 12 for ~2.4x mod, not sure about what diff 16 multiplier is.
Amulet and speed 00:00
rolling magma bug 00:15
ball lightning forgot to swap bow 00:35
rolling magma 00:42
ethereal knives 00:52
eye of winter 01:00
creeping frost 01:16
spark 02:09
forbidden rite 02:21
blazing salvo 02:31
fireball 02:58
iceshot 03:35
ice spear 03:51
rain of arrows and toxic rain 04:20
lightning strike 04:54
galvanic arrow 05:23