Back in 2022 libtorrent implemented memory mapped IO as part of v2 [1]. Unfortunately, it didn't go so well. Memory usage went through the sky, leading to performance degradation and crashing [2]. The issue is still open in the project to this day, and many programs have stuck with the v1.2 library instead.

It looks like they are headed to a multi-threaded pread implementation now [3] and someone has created a patch to tweak the current mmap fallback that uses pread to perform better in the meantime [4].

[1] - https://www.libtorrent.org/upgrade_to_2.0-ref.html

[2] - https://github.com/arvidn/libtorrent/issues/6667

[3] - https://github.com/arvidn/libtorrent/pull/7013

[4] - https://github.com/qbittorrent/qBittorrent/pull/21300