2d sprites, if I'm not being an idiot here, don't really use VRAM
This is incorrect, it's that our 2D sprites require the CPU to do work on textures each frame, which the PS3 doesn't handle well if you want to fill up VRAM with textures.
why can something like the last of us or tomb raider run on a ps3 but skullgirls and now indivisible strain the system?
As was already mentioned, Last of Us and Tomb Raider run at 30 FPS (or under) on PS3. SG runs at 60, mostly. That means each frame on SG has half the time to do everything it needs to do.
If SG ran at 30? No problems ever, and would have saved me a ton of work! Except that it would feel like BUTTASS.
For a real-world comparison - if you run at 1080p on PS3 (forced, by disabling 720 in the PS3's display settings), put Peacock/Big Band vs Peacock/Big Band, drop 2 Lennys and tag in both Bands, then have 1 Band do Tympany to the other while both calling Peacock...that's around 48-46 FPS on PS3...and it looks and feels like CRAP compared to 60, doesn't it? :^P
Roughly speaking:
Each time you want a
2D character to look different, it's a new drawing. If the sprites are large a new drawing takes a lot of space.
In 3D, the model takes up a lot of space with textures etc, but AFTER you make the model, each time you want a
3D character to look different it is a new animation frame stored as joint transformations, which is a fancy way of saying "it's some numbers in a row".
Once you get to fighting game character complexity / number of animations, properly-compressed 3D takes up
much less space than compressed 2D. And adding a new animation to a 3D character takes up MUCH less space than adding a new animation to a 2D character. One animation
frame on Skullgirls compresses to an average of 60-80K. (Big Band's frames can be upwards of 200K.) An entire 3D ANIMATION can fit in 80K.
Skullgirls fits 6 high-res characters in a space that BlazBlue could only fit 2 - and they had to take out frames from Tager when they gave him Gadget Finger in CS.
To do that, it does a lot of CPU decompression for textures, each frame.
Indivisible displays more sprites on average per frame than SG, with enemies walking around and stuff. It also will need to support bigger sprites than SG ever had.
This means more CPU work, which means the VRAM / CPU RAM separation on PS3 is an even bigger cause of slowdown.
3D games like Last of Us do the following (generally):
Put a bunch of textures in VRAM, once. Each frame, send new vertices to the graphics card which reuses existing textures to render 3D polygons. This means putting textures in VRAM can be slow and that's fine for 3D games.
Skullgirls updates the sprite texture per frame, because it needs to decompress the sprite data (which is big, remember, so it has to be compressed) on the CPU. Which means SG has to deal with that same slowness, each frame, and work around it. I'm continually surprised it runs at 60 at all! :^P
Doing palettization (that thing that changes your 1P color to 2P color) in shaders is also super expensive, because modern hardware got rid of that built-in ability. Old hardware had the ability to specify that a texture was palettized and pass it a palette and the hardware would do it for you! Oh well...
For comparison, modern graphics cards have the 3D lighting equation (very complicated) built in as a single instruction, because it is used commonly, which saves a massive amount of time for 3D. SG can't use that at all.