the scale2x algorithms are only meant to run once during initing...and thats it. I dont see how that would impact performance in OGL for a bigger texture size thats still less than 4096x4096 in size (otherwise, software mode kicks in...)
Sounds good, but it doesn't work.
The thing with emulators is, there's no such thing as an initialization phase where all graphics are initialized: most of the times you don't know what the game is going to look like until the game runs.
In the first JEmu I did some filtering per tile to make things look more smooth and gain some performance, but that really didn't look good. One reason is that when you have one tile, the filtering that needs to be done on the edges of one tile depends on the neighbouring tiles. So you end up with nicely filtered tiles with very obvious edges.
And then there's also dynamic palettes, transparency, even tiles/sprites that are in RAM (so dynamic too) etc. so the right thing to do it, afaik, is to filter (scale2x, averaging, whatever) the final image.