The MiSTer core has an extremely large OAM implementation that does not mirror the hardware at all. It wastes a ton of space and does simultaneous access to many addresses at once, requiring it to be implemented as a massive register instead of RAM. This consumes around 27% of the Pocket's resources.

This was introduced in MiSTer-devel/NES_MiSTer@922cdcb, which added extra sprite functionality to the core.
The MiSTer core has an extremely large OAM implementation that does not mirror the hardware at all. It wastes a ton of space and does simultaneous access to many addresses at once, requiring it to be implemented as a massive register instead of RAM. This consumes around 27% of the Pocket's resources.
This was introduced in MiSTer-devel/NES_MiSTer@922cdcb, which added extra sprite functionality to the core.