You don't have a lot of time in the VBlank to decide what tiles you're moving to VRAM.
I haven't developed an NES game before but I would assume you'd need to decode your game's map data into a buffer out of VBlank then stream it in after updating OAM. Unless you have the luxury of being able to just read it directly from ROM and write it to VRAM without a lot of intermediate processing.
I think most NES games had various schemes to represent the map data (compression schemes basically) with Metroid/Kid Icarus being a particularly elaborate one (a game editor "MetEdit" out there explains it). Not sure how they split the unpacking/VRAM writing workload.
I haven't developed an NES game before but I would assume you'd need to decode your game's map data into a buffer out of VBlank then stream it in after updating OAM. Unless you have the luxury of being able to just read it directly from ROM and write it to VRAM without a lot of intermediate processing.
I think most NES games had various schemes to represent the map data (compression schemes basically) with Metroid/Kid Icarus being a particularly elaborate one (a game editor "MetEdit" out there explains it). Not sure how they split the unpacking/VRAM writing workload.