The technical challenge is that a Crystal program needs to be inspected as a whole. Simplified, changes in location A can have effects on some completely unrelated location B. That makes it hard to cache intermediary results and a semantic analysis needs to cover the entire program (including the standard library), not just the files that were changed since the last time.

This applies to the responsiveness and memory consumption of the language server as well as the regular compilation process. So this is an important topic, and we're working on improvements. It's a complex topic, but there are some ideas.