diff --git a/pysoot/lifter.py b/pysoot/lifter.py
index 79c6352..e9de99a 100644
--- a/pysoot/lifter.py
+++ b/pysoot/lifter.py
@@ -88,11 +88,16 @@ def _get_ir(self):
         # delayed import
         from .soot_manager import SootManager
 
-        self.soot_wrapper = SootManager()
+        soot_wrapper = SootManager()
 
         log.info("Running Soot with the following config: " + repr(config))
-        self.soot_wrapper.init(**config)
-        self.classes = self.soot_wrapper.get_classes()
+        soot_wrapper.init(**config)
+        self.classes = soot_wrapper.get_classes()
+        self._hierarchy = soot_wrapper.compute_hierarchy()
+
+    def getSubclassesOf(self, class_name: str) -> list[str]:
+        """Return pre-computed subclasses of the given class name."""
+        return self._hierarchy.get(class_name, [])
 
 
 def _get_java_home() -> str:
diff --git a/pysoot/soot_manager.py b/pysoot/soot_manager.py
index 441bd6f..6895502 100644
--- a/pysoot/soot_manager.py
+++ b/pysoot/soot_manager.py
@@ -89,8 +89,15 @@ def get_classes(self):
                 classes[soot_class.name] = soot_class
         return classes
 
-    def getSubclassesOf(self, class_name: str) -> list[str]:
-        return [
-            c.getName()
-            for c in self.hierarchy.getSubclassesOf(self.class_name_map[class_name])
-        ]
+    def compute_hierarchy(self) -> dict[str, list[str]]:
+        """Pre-compute subclass relationships for all classes."""
+        result = {}
+        for name, raw_class in self.class_name_map.items():
+            try:
+                result[name] = [
+                    c.getName() for c in self.hierarchy.getSubclassesOf(raw_class)
+                ]
+            except Exception:
+                # Some classes (e.g. interfaces) may not support getSubclassesOf
+                pass
+        return result
diff --git a/tests/test_pysoot.py b/tests/test_pysoot.py
index 799d802..e8d39e0 100755
--- a/tests/test_pysoot.py
+++ b/tests/test_pysoot.py
@@ -76,7 +76,7 @@ def test_hierarchy(self):
         # Only check application classes — JDK classes (e.g. java.lang.System)
         # are phantom refs on modular JDKs (Java 9+) and won't appear in the hierarchy.
         test_subc = ["simple2.Class2", "simple2.Class1"]
-        subc = lifter.soot_wrapper.getSubclassesOf("java.lang.Object")
+        subc = lifter.getSubclassesOf("java.lang.Object")
         assert all([c in subc for c in test_subc])
 
     def test_exceptions1(self):
@@ -99,7 +99,7 @@ def test_exceptions1(self):
     def test_android1(self):
         apk = os.path.join(self.test_samples_folder, "android1.apk")
         lifter = Lifter(apk, input_format="apk", android_sdk=self.android_sdk_path)
-        subc = lifter.soot_wrapper.getSubclassesOf("java.lang.Object")
+        subc = lifter.getSubclassesOf("java.lang.Object")
         assert "com.example.antoniob.android1.MainActivity" in subc
         main_activity = lifter.classes["com.example.antoniob.android1.MainActivity"]