I think this issue is related to https://gitlab.com/ubports/development/core/lomiri-app-launch/-/merge_requests/47#note_1567842127. As I understand from the discussion, if lomiri-app-launch is unable to write oom_adj values, it will attempt to spawn an OOM SUID helper, and doing so for every thread takes lot of time/resources.
The easiest workaround for now is to revert Android commits in the device port kernel that made read-only oom_adj (as they had been also discarded upstream):
https://gitlab.com/ubports/porting/community-ports/android10/xiaomi-redmi-note-9-pro/kernel-xiaomi-sm6250/-/commit/d13041168eae01e79e9625f280a640ab8f960033
https://gitlab.com/ubports/porting/community-ports/android10/xiaomi-redmi-note-9-pro/kernel-xiaomi-sm6250/-/commit/c45ed3b1cdf7d8e485bb4b50dfd1ec9842b4bd64