Gameboy Development Forum

Well, it is as expensive as the overhead of the CPU instructions used to switch between the banks.

It could be as simple as...

    ld A,5
    ld [$2000],A

But in practice you also need to...
- Potentially keep track of which bank was previously selected, if you need to be able to return to that bank later. This is applicable if you have code in a selectable bank that can call other code in bank 0 to load data from another bank.
- Calculate which bank you want to read from.
- Calculate which other bank you want to read from. (For example if one bank contains metatile data, and the other bank contains the corresponding tile data.)

In general, you would need to actually benchmark/calculate the worst case runtime for your code, for example using breakpoints in BGB. It' may also be a good idea to plan ahead and make it possible to load data over multiple frames, ahead of scrolling in a certain direction.

For loading bulk data (ie loading a new level/screen) it's generally recommended to disable the LCD, which gives you uninterrupted access to VRAM. The screen will go blank during this time, but this generally isn't a problem during a screen transition.