diff --git a/synapse/config/cache.py b/synapse/config/cache.py
index 403893a3d3..a8dc93c225 100644
--- a/synapse/config/cache.py
+++ b/synapse/config/cache.py
@@ -103,17 +103,11 @@ class CacheConfig(Config):
         # Set the global one so that it's reflected in new caches
         properties.default_factor_size = self.global_factor
 
-        # Load cache factors from the environment, but override them with the
-        # ones in the config file if they exist
-        individual_factors = {
-            key[len(_CACHE_PREFIX) + 1 :].lower(): float(val)
-            for key, val in self._environ.items()
-            if key.startswith(_CACHE_PREFIX + "_")
-        }
-        individual_factors_config = cache_config.get("per_cache_factors", {}) or {}
-        if not isinstance(individual_factors_config, dict):
+        # Load cache factors from the config, but override them with the
+        # environment if they exist
+        individual_factors = cache_config.get("per_cache_factors", {}) or {}
+        if not isinstance(individual_factors, dict):
             raise ConfigError("caches.per_cache_factors must be a dictionary")
-        individual_factors.update(individual_factors_config)
 
         for cache, factor in individual_factors.items():
             if not isinstance(factor, (int, float)):
@@ -122,6 +116,15 @@ class CacheConfig(Config):
                 )
             self.cache_factors[cache.lower()] = factor
 
+        # Override with environment
+        individual_factors.update(
+            {
+                key[len(_CACHE_PREFIX) + 1 :].lower(): float(val)
+                for key, val in self._environ.items()
+                if key.startswith(_CACHE_PREFIX + "_")
+            }
+        )
+
         # Resize all caches (if necessary) with the new factors we've loaded
         self.resize_all_caches()
 
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 296706a396..cd48262420 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -90,13 +90,27 @@ class Cache(object):
 
     def __init__(
         self,
-        name,
-        max_entries=1000,
-        keylen=1,
-        tree=False,
-        iterable=False,
-        apply_cache_factor_from_config=True,
+        name: str,
+        max_entries: int = 1000,
+        keylen: int = 1,
+        tree: bool = False,
+        iterable: bool = False,
+        apply_cache_factor_from_config: bool = True,
     ):
+        """
+        Args:
+            name: The name of the cache
+            max_entries: Maximum amount of entries that the cache will hold
+            keylen: The length of the tuple used as the cache key
+            tree: Use a TreeCache instead of a dict as the underlying cache type
+            iterable: If True, count each item in the cached object as an entry,
+                rather than each cached object
+            apply_cache_factor_from_config: Whether cache factors specified in the
+                config file affect `max_entries`
+
+        Returns:
+            Cache
+        """
         cache_type = TreeCache if tree else dict
         self._pending_deferred_cache = cache_type()
 
diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py
index 293a3f172a..29fabac3cd 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -15,6 +15,7 @@
 
 import threading
 from functools import wraps
+from typing import Callable, Optional, Type, Union
 
 from synapse.config import cache as cache_config
 from synapse.util.caches.treecache import TreeCache
@@ -52,18 +53,18 @@ class LruCache(object):
 
     def __init__(
         self,
-        max_size,
-        keylen=1,
-        cache_type=dict,
-        size_callback=None,
-        evicted_callback=None,
-        apply_cache_factor_from_config=True,
+        max_size: int,
+        keylen: int = 1,
+        cache_type: Type[Union[dict, TreeCache]] = dict,
+        size_callback: Optional[Callable] = None,
+        evicted_callback: Optional[Callable] = None,
+        apply_cache_factor_from_config: bool = True,
     ):
         """
         Args:
-            max_size (int):
+            max_size: The maximum amount of entries the cache can hold
 
-            keylen (int):
+            keylen: The length of the tuple used as the cache key
 
             cache_type (type):
                 type of underlying cache to be used. Typically one of dict
@@ -76,7 +77,7 @@ class LruCache(object):
                 entry
 
             apply_cache_factor_from_config (bool): If true, `max_size` will be
-                multiplied by a cache factor derived from the homeserver config.
+                multiplied by a cache factor derived from the homeserver config
         """
         cache = cache_type()
         self.cache = cache  # Used for introspection.