Extreme framerate drop when enabling lossless scaling

[regal imp]

Greetings.
I'm attempting to use lossless scaling with a dual-GPU setup. System specs are as follows:
OS: Windows 10 IoT LTSC version 10.0.19044 Build 19044
Motherboard: MSI X570-A Pro
CPU: AMD Ryzen 9 5900X
GPU: Nvidia GeForce RTX 3060 Ti (Rendering GPU)
GPU: AMD Radeon RX 6400
RAM: G.Skill Trident Z Neo 128 GB (4x32 GB), 3600 MHz
Monitors
Primary: Alienware AW3225QF 3840x2160 @ 120 Hz
Secondary: Titan Army P27A2R 2560x1440 @ 120 Hz
Both monitors are plugged into the RX 6400, primary with an HDMI 2.1 cable, and secondary with a DisplayPort 2.1 cable.
I've already made the necessary registry edits to be able to select which GPU to render a game with.
The issue occurs when I start scaling. The game I'm using for testing purposes is MechWarrior 5: Mercenaries.
Upon beginning scaling, FPS drops from ~46 to ~12 and the input lag skyrockets to 100+, from a base of ~20. The game becomes very choppy and laggy. I don't know if my settings for this game are the issue, or if it's a hardware issue. Any help would be much appreciated.

[shy carbon]

Are you using HDR? What flow scale setting? performance mode or no? queue target, max frame latency settings in LS?

I would guess that the most likely issues are:

• 1: PCI-E BW. The 6400 only supports 4.0 x4 pci-e, this can apparently handle about 100fps at 4k, but that would be halved if HDR is enabled. Halved again if it's on 3.0 x4 for some reason (check it with a tool like gpu-z or hwinfo).

• 2: Insufficient GPU performance on the 6400, check the GPU load % (should be under 80%), try lower flowscale (like 50%) and performance mode if it's too high.

Might also be an issue with queue size or max frame latency being too low.

@regal imp

One other thing, for pci-e BW i am not sure how 10-bit color behaves. Worth a quick try with 8 if that's the issue to see if there's radical change

[regal imp]

I'm using HDR (DisplayHDR True Black). Flow scale setting 50%. Performance mode in both sections off. Queue target 1, max frame latency 3.
The 6400 is running at PCIe 4.0x4.
The load drops to under 10% when scaling is initiated. It's like the GPU becomes inactive and isn't attempting to generate frames at all. The moment I stop scaling, its usage shoots up to ~40%, and the usage of the 3060 Ti drops from 100% to 80-90. I also tried turning on performance mode in both categories and it changed nothing.
As for 8-bit color, I have no idea what you're talking about.

[shy carbon]

this setting in nvidia control panel

I think you're just severely pci-e bottlenecked, but give it a try without HDR (disable "hdr support" in LS too) and with 8 bit color. If it improves radically, it's pci-e.