poor performance on fractional scaling on Linux Mint

I do have a laptop with an Intel/NVidia optimus graphics combo. I'm using "on-demand" mode using sudo prime-select on-demand. The attached LCD screen is a 32" offering up to 3840 x 2160 resolution. If using a screen scaling setting of 100% it as fast as working on the laptop's local LCD. However if I enable fractional scaling to e.g. 125% (which offers the best usability for me) graphics performance degrades significantly. I've used glxgears to record the frame rate for full screen mode: getting 60 fps for 100% scaling and only ~25 fps for 125% scaling. Any ideas how to fix this? some specs:

$> sudo inxi -G -v 2
System:
  Host: p43s Kernel: 5.15.0-56-generic x86_64 bits: 64
    Desktop: Cinnamon 5.6.5 Distro: Linux Mint 21.1 Vera
Machine:
  Type: Laptop System: LENOVO product: 20RH0016GE v: ThinkPad P43s
    serial: 
  Mobo: LENOVO model: 20RH0016GE v:  WIN serial: 
    UEFI: LENOVO v: N2IETA0W (1.78 ) date: 10/14/2022
Battery:
  ID-1: BAT0 charge: 41.9 Wh (100.0%) condition: 41.9/50.5 Wh (83.1%)
CPU:
  Info: quad core Intel Core i7-8665U [MT MCP] speed (MHz): avg: 656
    min/max: 400/4800
Graphics:
  Device-1: Intel WhiskeyLake-U GT2 [UHD Graphics 620] driver: i915 v: kernel
  Device-2: NVIDIA GP108GLM [Quadro P520] driver: nvidia v: 525.60.11
  Device-3: Logitech Logitech Webcam C925e type: USB
    driver: snd-usb-audio,uvcvideo
  Device-4: Chicony ThinkPad T490 Webcam type: USB driver: uvcvideo
  Display: server: X.Org v: 1.21.1.3 driver: X: loaded: modesetting,nvidia
    unloaded: fbdev,nouveau,vesa gpu: i915 resolution: 6144x3456~60Hz
  OpenGL: renderer: Mesa Intel UHD Graphics 620 (WHL GT2)
    v: 4.6 Mesa 22.0.5
...
Info:
  Processes: 374 Uptime: 1d 2m Memory: 46.81 GiB used: 8.19 GiB (17.5%)
  Shell: Zsh inxi: 3.3.13