diff --git a/lab/tools.py b/lab/tools.py
index 9cd7d2034..9493987cc 100644
--- a/lab/tools.py
+++ b/lab/tools.py
@@ -13,19 +13,7 @@
 import sys
 from pathlib import Path
 
-# Use simplejson where it's available, because it is compatible (just separately
-# maintained), puts no blanks at line endings and loads json much faster:
-# json_dump: 44.41s, simplejson_dump: 45.90s
-# json_load: 7.32s, simplejson_load: 2.92s
-# We cannot use cjson or ujson for dumping, because the resulting files are
-# hard to read for humans (cjson_dump: 5.78, ujson_dump: 2.44). Using ujson for
-# loading might be feasible, but it would only result in a very small speed gain
-# (ujson_load: 2.49). cjson loads even slower than simplejson (cjson_load: 3.28).
-try:
-    import simplejson as json
-except ImportError:
-    import json
-
+import orjson
 
 DEFAULT_ENCODING = "utf-8"
 
@@ -260,19 +248,11 @@ def compute_log_score(success, value, lower_bound, upper_bound):
 
 
 class Properties(dict):
-    class _PropertiesEncoder(json.JSONEncoder):
-        def default(self, o):
-            if isinstance(o, Path):
-                return str(o)
-            else:
-                return super().default(o)
-
-    JSON_ARGS = {
-        "cls": _PropertiesEncoder,
-        "indent": 2,
-        "separators": (",", ": "),
-        "sort_keys": True,
-    }
+    def default(self, obj):
+        if isinstance(obj, Path):
+            return str(obj)
+        else:
+            return obj
 
     """Transparently handle properties files compressed with xz."""
 
@@ -289,14 +269,21 @@ def __init__(self, filename=None):
         dict.__init__(self)
 
     def __str__(self):
-        return json.dumps(self, **self.JSON_ARGS)
+        return self.to_bytes().decode()
+
+    def to_bytes(self):
+        return orjson.dumps(
+            self,
+            default=self.default,
+            option=orjson.OPT_INDENT_2 | orjson.OPT_SORT_KEYS,
+        )
 
     def load(self, filename):
         path = Path(filename)
         open_func = lzma.open if path.suffix == ".xz" else open
-        with open_func(path) as f:
+        with open_func(path, "rb") as f:
             try:
-                self.update(json.load(f))
+                self.update(orjson.loads(f.read()))
             except ValueError as e:
                 logging.critical(f"JSON parse error in file '{path}': {e}")
 
@@ -308,8 +295,8 @@ def write(self):
         assert self.path
         self.path.parent.mkdir(parents=True, exist_ok=True)
         open_func = lzma.open if self.path.suffix == ".xz" else open
-        with open_func(self.path, "w") as f:
-            json.dump(self, f, **self.JSON_ARGS)
+        with open_func(self.path, "wb") as f:
+            f.write(self.to_bytes())
 
 
 class RunFilter:
diff --git a/setup.py b/setup.py
index 235158c44..44d04ee6e 100644
--- a/setup.py
+++ b/setup.py
@@ -36,7 +36,7 @@
     ],
     install_requires=[
         "matplotlib",  # for scatter plots
-        "simplejson",  # optional, speeds up reading properties files
+        "orjson",
         "txt2tags>=3.6",  # for HTML and Latex reports
     ],
     python_requires=">=3.7",