Libtorrent 2.0 switched exclusively to mmaped read/writes for torrent downloads, which resulted in various performance and especially memory consumption issues on ALL platforms.
For some reason Windows handled increased memory consumption the least gracefully.
Many people continued to use v1.2 which use regular files.
V2.1 ended up using pread/pwrite nowz it's fine now.
The issue continued for 3 years more or less.