diff --git a/proxywatchd.py b/proxywatchd.py
index 0940391..bcaeb23 100644
--- a/proxywatchd.py
+++ b/proxywatchd.py
@@ -239,11 +239,20 @@ class ThreadScaler(object):
         return 'threads=%d queue=%d target_per_thread=%d' % (
             current_threads, queue_size, self.target_queue_per_thread)
 
-class ProxyTestState():
+class ProxyTestState(object):
     """Thread-safe state for a proxy being tested.
 
     Holds test results and evaluates final pass/fail status.
     """
+    __slots__ = (
+        'ip', 'port', 'proxy', 'auth', 'proto', 'failcount', 'checktime',
+        'success_count', 'total_duration', 'country', 'mitm', 'consecutive_success',
+        'asn', 'isoldies', 'completion_queue', 'lock', 'results', 'completed',
+        'evaluated', 'last_latency_ms', 'exit_ip', 'reveals_headers',
+        'last_fail_category', 'original_failcount', 'had_ssl_test', 'ssl_success',
+        'cert_error'
+    )
+
     def __init__(self, ip, port, proto, failcount, success_count, total_duration,
                  country, mitm, consecutive_success, asn=None, oldies=False,
                  completion_queue=None, proxy_full=None):
@@ -433,12 +442,14 @@ class ProxyTestState():
                 return (False, fail_category)
 
 
-class TargetTestJob():
+class TargetTestJob(object):
     """Job to test a single proxy against a single target.
 
     Multiple TargetTestJob instances share the same ProxyTestState,
     allowing tests to be interleaved with other proxies in the queue.
     """
+    __slots__ = ('proxy_state', 'target_srv', 'checktype', 'worker_id')
+
     def __init__(self, proxy_state, target_srv, checktype, worker_id=None):
         self.proxy_state = proxy_state
         self.target_srv = target_srv