From early in the season:
“ The Mercedes specification is bulkier in the area closest to the floor, which limits airflow through the AMR23’s diffuser. This area is designed for a car with ultra-compact bodywork like the W14, as seen with the Mercedes ‘zeropods.’
Having a less efficient diffuser means the AMR23 needs more rear wing to compensate, which generates more drag.”
https://formu1a.uno/en/how-the-mercedes-gearbox-has-slowed-down-aston-martin/
And the fact that they have barely any cap space going into next year…