Add developer notes about blocking GUI code

Author: ryanofsky

doc/developer-notes.md

@@ -625,6 +625,19 @@ GUI
     should not interact with the user. That's where View classes come in. The converse also
     holds: try to not directly access core data structures from Views.
 
+- Avoid adding slow or blocking code in the GUI thread. In particular do not
+  add new `interface::Node` and `interface::Wallet` method calls, even if they
+  may be fast now, in case they are changed to lock or communicate across
+  processes in the future.
+
+  Prefer to offload work from the GUI thread to worker threads (see
+  `RPCExecutor` in console code as an example) or take other steps (see
+  https://doc.qt.io/archives/qq/qq27-responsive-guis.html) to keep the GUI
+  responsive.
+
+  - *Rationale*: Blocking the GUI thread can increase latency, and lead to
+    hangs and deadlocks.
+
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