Ok, I got a similar recreation happen
Main Menu, 88-95fps
Online Arena Menu 60fps
Selecting MA started a DLa updated map. 60fps dropped to 9fps during DL.
In MA Tower it was 66fps, I waited for a few minutes no real change,
In flight it went to 116-120fps.
I closed AH and went through it again. No map DL tho
Main Menu 93fps
Online Arena Menu 60fps
In MA Tower was 95fps, no change.
In flight 120+fps .
There is a FPS stair-stepping depending on what's happening/seen by GPU.
The fact your new PC runs AH with no flaws, *suggest* the older PC has something going on in the background or a possible GPU setting.
If there is nothing sticking out in Task Manager lets try GPU.
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1) Game Mode should be enabled
2) Change the Power Plan
Power plans in Windows 10 control how your PC consumes/saves power. Microsoft introduced a power plan called Ultimate Performance for workstations, which can provide a performance boost.
By default this power plan is a hidden feature, but you can use the following steps to enable it:
1) Open the Run dialog “Window key + R”, or search in Start Menu. Type or copy and paste powercfg.cpl and press Enter.
2) Select Ultimate Performance. If you don’t see this power plan, simply continue to the next step to unhide it.
3) Open Start Menu, type cmd in the search bar. Select Command Prompt and Run as Administrator.
4) In the Command Prompt, type or paste the following command and press Enter.
powercfg -duplicatescheme e9a42b02-d5df-448d-aa00-03f14749eb61
If you see a prompt similar to this, return to step 2 to enable the Ultimate Performance power plan.
Hardware-Accelerated GPU Scheduling under Advanced graphics settings (or “Default graphics settings” for Windows 10) toggle On/Enable Hardware-Accelerated GPU Scheduling.
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Make sure the GPU resolution matches the game and monitor resolution.
Nvidia Control Panel
Texture filtering - should be disabled for AH, it's not used in older games. If you play other games on it. You can go to
V-Sync - Use the application setting (to use the settings within the application to control V-Sync), Fast (improves latency without introducing tearing).
Off (useful if performance is more important than image quality).
I would try Fast first.
To turn off the NVIDIA GeForce Experience overlay,
open the application, navigate to Settings (gear icon), and toggle off the In-Game Overlay option.
Make sure no screen capture is running
NVIDIA screen capture is powered by the NVIDIA App or GeForce Experience, allowing for high-quality gameplay recording, streaming, and screenshots with *minimal* performance impact. Users can access the overlay via Alt+Z to configure settings, while Alt+F9 toggles recording, Alt+F10 saves instant replays, and Alt+F1 captures screenshots.
Then lets see if we woke anything up. If not we can go deeper. Keep it simple for now.
Lets us know the results.