If you don't mind my asking, why the single big file in the first place? I read somewhere that it streamlines the loading process, but it sounds like the console version's method doesn't do that and is faster.
Unpacked, Path of Exile is hundreds of thousands of tiny files. Many of them are tiny hand edited or tool generated text files that might just be 1KB or so. Packing them together is important for performance so you don't have to do so many disk seeks while loading files.
But so long as your files are still fairly large (32 MB or so per file for example) then the performance of seperate files is totally fine.
For conveniance of the original patcher, it was simple to have one big file. And it was a fairly common thing to do at the time ~13 years ago. Our patcher was modeled on the Guild Wars 1 patcher, and it's quite similar, but blizzard had it's MPQ files and so on. The big single file is structured such that it's very easy for us to work out the minimum set of files that need to be patched. We just check a single hash if nothing has changed, and only explore the file tree to the parts that have changed if not.
We didn't add steam support until the game was already out.
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