The main thing I want is actually rather simple. It's just to have a program that can write and edit GF grammars.
Whether I am manipulating source code, PGF files or something in-between doesn't in itself matter to me, I'll go with whatever's the easiest and cleanest. The only catch is that it can't take too long to compile after each small edit, hence my initial question.
When the program is in active use, and an edit to the grammar is made (say, the word order in a certain construction is changed) I want to have access to a compiled grammar as soon as possible which can translate and generate with the new changes. But it wouldn't need to have access to all lexical entries for this, only at least one of each base category (a noun for each noun gender, a verb for each declension etc).
When editing finishes, the program is free to compile everything and take its time doing so.
I also wasn't talking about anything as sophisticated as splitting or merging PGFs, I could simply generate two source-codes (one minimal, one full), merge those when needed, then compile them separately. I'm open to suggestions though. But no I'm not looking to write my own compiler or optimise at all costs, it just needs to work and be usable (and preferably readable and maintainable) and I'll be happy :)
Hopefully that clears some things up,