Low CPU utlization, FPS gradually dropping throughout play - forcing game restart

i9-14900HX / 4070 laptop GPU / 32GB RAM.
Latest NVIDIA drivers (580.97), and Windows 11 up to date, and in performance mode.

The PC really should be able to handle the game.

Even at low graphics settings, dumping all settings with high CPU impact, yet after about one match my FPS has dropped from around 70 down to 40 (remains so even on the menu!). GPU utilization low. The performance overlay seems to indicate the CPU is the bottleneck – but I'm only seeing 30% utilization (see screenshot).

(in-game performance monitor in corner)

Restarting the game improves FPS a lot (back up to 70) but only for a short while.

• Playing through Steam, and Steam Overlay is enabled.
• Tested with Windows game mode on and off (no change).
• Tried both windows and fullscreen mode (no change).

Anyone have any ideas?