diff --git a/massive/rest/__init__.py b/massive/rest/__init__.py
index 5a00da5a..0fc42de3 100644
--- a/massive/rest/__init__.py
+++ b/massive/rest/__init__.py
@@ -54,6 +54,9 @@ def __init__(
         connect_timeout: float = 10.0,
         read_timeout: float = 10.0,
         num_pools: int = 10,
+        # Per-host connection limit. 10 rather than urllib3's default of 1 so that
+        # threaded clients work sensibly out of the box; see BaseClient.
+        maxsize: int = 10,
         retries: int = 3,
         base: str = BASE,
         pagination: bool = True,
@@ -66,6 +69,7 @@ def __init__(
             connect_timeout=connect_timeout,
             read_timeout=read_timeout,
             num_pools=num_pools,
+            maxsize=maxsize,
             retries=retries,
             base=base,
             pagination=pagination,
@@ -78,6 +82,7 @@ def __init__(
             connect_timeout=connect_timeout,
             read_timeout=read_timeout,
             num_pools=num_pools,
+            maxsize=maxsize,
             retries=retries,
             base=base,
             pagination=pagination,
diff --git a/massive/rest/base.py b/massive/rest/base.py
index 3349d7ef..05c5694a 100644
--- a/massive/rest/base.py
+++ b/massive/rest/base.py
@@ -34,6 +34,7 @@ def __init__(
         verbose: bool,
         trace: bool,
         custom_json: Optional[Any] = None,
+        maxsize: int = 10,
     ):
         if api_key is None:
             raise AuthError(
@@ -75,6 +76,11 @@ def __init__(
         # https://urllib3.readthedocs.io/en/stable/reference/urllib3.connectionpool.html#urllib3.HTTPConnectionPool
         self.client = urllib3.PoolManager(
             num_pools=num_pools,
+            # Connections kept per HOST. urllib3 defaults this to 1, which serialises
+            # any threaded client onto a single connection and renegotiates TLS for
+            # every request that cannot get it. num_pools does not help: it caps the
+            # number of distinct host pools, and this client talks to one host.
+            maxsize=maxsize,
             headers=self.headers,  # default headers sent with each request.
             ca_certs=certifi.where(),
             cert_reqs="CERT_REQUIRED",
diff --git a/test_rest/test_connection_pool.py b/test_rest/test_connection_pool.py
new file mode 100644
index 00000000..f8f81196
--- /dev/null
+++ b/test_rest/test_connection_pool.py
@@ -0,0 +1,22 @@
+import unittest
+
+from massive import RESTClient
+
+
+class ConnectionPoolTest(unittest.TestCase):
+    def test_maxsize_defaults_to_ten(self):
+        client = RESTClient(api_key="test")
+
+        assert client.client.connection_pool_kw["maxsize"] == 10
+
+    def test_maxsize_is_configurable(self):
+        client = RESTClient(api_key="test", maxsize=64)
+
+        assert client.client.connection_pool_kw["maxsize"] == 64
+
+    def test_maxsize_is_independent_of_num_pools(self):
+        # num_pools caps the number of distinct HOST pools, which does not help a client
+        # that talks to one host from many threads -- that is what maxsize controls.
+        client = RESTClient(api_key="test", num_pools=50, maxsize=32)
+
+        assert client.client.connection_pool_kw["maxsize"] == 32