diff --git a/.changeset/tiny-boxes-play.md b/.changeset/tiny-boxes-play.md
new file mode 100644
index 000000000..d0bd3fd00
--- /dev/null
+++ b/.changeset/tiny-boxes-play.md
@@ -0,0 +1,9 @@
+---
+"mobx": patch
+"eslint-plugin-mobx": patch
+"mobx-react": patch
+"mobx-react-lite": patch
+"mobx-undecorate": patch
+---
+
+Disable `useDefineForClassFields` in `tsconfig.json` to reduce memory footprint and improve performance of class instances with undefined and unused members.
diff --git a/tsconfig.json b/tsconfig.json
index 4be5dff7e..b9fb5f3b3 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -16,7 +16,8 @@
         "noImplicitThis": false,
         "noEmit": true,
         "experimentalDecorators": true,
-        "useDefineForClassFields": true,
+        // See: https://github.com/mobxjs/mobx/pull/3884
+        "useDefineForClassFields": false,
         "jsx": "react",
         "esModuleInterop": true
     },
diff --git a/tsconfig.test.json b/tsconfig.test.json
index f8e4f8c91..0bfdfaf81 100644
--- a/tsconfig.test.json
+++ b/tsconfig.test.json
@@ -3,6 +3,8 @@
     "compilerOptions": {
         // "module": "commonjs",
         "allowJs": true,
-        "noUnusedLocals": false
+        "noUnusedLocals": false,
+        // See: https://github.com/mobxjs/mobx/pull/3884
+        "useDefineForClassFields": true
     }
 }