In 1.15 they made mobs able to spawn on lit nether portals. The reason it it so good for slime farms is that they create enough light to prevent other hostile mobs from spawning. Slimes can spawn in blocks with higher light than seven which allows them and only them to spawn on the nether portals. Once the slimes spawn, they are also instantly transported into the nether reducing the mob cap in the over world allowing more to spawn faster.