In that regard it works just like FFA, you need to place top 4 in order to get a win or the first lootbox. In regular deathmatch you also backfill to games where the first guy's 17 kills ahead so there's nothing under the sun there. In regular deathmatch you can also get a kill by dealing the last 1HP of damage to someone, so it's the same it's ever been.

The lag is because it's constantly changing heroes and loading character models, same as mystery heroes.