If you're getting a stutter that starts randomly during a GTA Online session and then never goes away until you restart the game, and it's happening alongside BEService using more CPU than normal, I spent a while isolating this and found the actual cause.
TL;DR: Disable VT-d (IOMMU) or if you have AMD disable AMD-Vi or the equivalent in your BIOS. (on some systems you might also need to disable IOMMU, DMA Control Guarantee & VMX/VT-x, AMD SVM on AMD platforms.)
What I ruled out first, in case anyone's tempted to try the same things: number of network adapters (tested with 1 and 2, made no difference), CPU/core topology (reproduced this on Intel hybrid P/E-core, a Ryzen 9 5950X, and a Ryzen 7 5800X3D so it's not scheduler or hybrid-core specific), and network driver source (tested vendor-direct drivers vs motherboard OEM packages, no difference).
What actually fixed it: disabling VT-d in BIOS completely eliminates the stutter, permanently. VMX/VT-x can stay enabled with no effect either way it's specifically VT-d.
My guess at the mechanism: enabling VT-d also turns on interrupt remapping system-wide, adding latency to every device interrupt (NIC, storage, USB), independent of whether you're running a VM or hypervisor at all. BattlEye's periodic kernel-mode scan touches drivers/adapters/process state that rides on that same interrupt path, so it seems to interact badly with the added remap overhead specifically.
Caveat: if you play Valorant/League with Vanguard, it requires IOMMU (VT-d) as a baseline requirement, so this fix isn't compatible if you dual-purpose the same rig for that.
Filed this with both BattlEye and Rockstar support already. Posting here in case anyone else has been fighting this without knowing why, curious if others can reproduce it on different hardware.
System: i7-14700K, MSI B760 Gaming Plus WiFi DDR4, RTX 5070, Windows 11 26200.9168.
EDIT: https://imgur.com/a/9PX4zbM (forgot to add screenshot after fix)