Suppose the IDE has the code in memory, it wants it compiled (or syntax checked, or ...). Currently, it needs to write out a file, then call the compiler, and pick up the result from the file system, and parse the result. Ideal, would be a programmer's API to do this. But a compilation service, with RPC calls would already be a major improvement.

Also, if you store code, intermediate results, dependencies, ..., in a database (could even be in memory), you can reuse these intermediate results. The dependency graph can guide you to decide what needs to be changed etc.