
In the Betrayal league content, there are 17 members of the Immortal Syndicate who can appear in areas that you play through. As with all other content in Path of Exile, it's important that the game assets for these enemies are loaded before you encounter them, otherwise your client will have to load them synchronously, which stalls it for a period of time.
Because multiple Betrayal encounters occur in an area, and the results of one encounter can affect which Syndicate members spawn in the subsequent encounters, it's not possible to know in advance which specific Syndicate members need to be loaded, so we have to preload all of them.
On most users' machines, this preloading is complete before any Syndicate members are encountered in the area, with no performance impact. However, on slower machines or ones with platter-based hard drives...
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