KB5127648 update

The slowdown surviving a clean installation means another reinstall is not the informative next step. It also does not prove that KB5127648 caused the issue: your comparison changed Windows versions and potentially drivers and configuration.
Choose one repeatable game scene and record resolution, graphics settings, frame rate, frame-time behaviour, and GPU driver version. Check Task Manager for unexpected CPU, GPU, disk, or background activity during the stutter. Compare one supported graphics-driver configuration at a time, using the hardware manufacturers’ official packages.
Temporarily compare with optional recording or overlay features closed, while keeping security protections enabled. Check whether several games are affected or only one, and whether the issue appears immediately or after sustained load.
Submit Feedback Hub feedback with the exact Windows build, hardware, driver versions, affected games, and reproduction steps. I cannot confirm a Microsoft-documented fix for this configuration, and removing security updates indefinitely is not a sound resolution.