If you are using map entities to form your level (tiles) use "tex_share" (dunno in lite-c) to preserve some video memory.
The same thing (I think) can be applied to models , in MED use an "external texture". Not shure about this , but I think it loads it only once , and makes the models smaller (this way they'll use less memory to load).

BTW , if you have problems with FPS with a lot of tiles , dont assign them a function. Use one global function to control/update tiles one by one. A lot of small functions drop the frame rate a lot. Atleast thats what I've experienced in the past

EDIT : Forgot to mention. Again not shure , but I think weaker light calculations also lower the map entity size , wich should decrease memory usage and increase FPS.