diff --git a/doc/changelog.rst b/doc/changelog.rst
index 8cbb2fb3..9e6dc75d 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -8,7 +8,12 @@ Upcoming version (not yet released)
 MJX
 ^^^
 
-1. Improved performance of getting and putting device data by using a faster numpy array serialization method.
+1. Improved performance of getting and putting device data.
+
+   - Use ``tobytes()`` for numpy array serialization, which is orders of magnitude faster than converting to tuples.
+   - Avoid reallocating host ``mjData`` arrays when array shapes are unchanged.
+   - Speed up calculation of ``mjx.ncon`` for models with many geoms.
+   - Avoid calling ``mjx.ncon`` in ``mjx.get_data_into`` when ``nc`` can be derived from ``mjx.Data``.
 
 Version 3.1.2 (February 05, 2024)
 -----------------------------------
diff --git a/mjx/mujoco/mjx/_src/collision_driver.py b/mjx/mujoco/mjx/_src/collision_driver.py
index c54278b2..721b4345 100644
--- a/mjx/mujoco/mjx/_src/collision_driver.py
+++ b/mjx/mujoco/mjx/_src/collision_driver.py
@@ -313,8 +313,16 @@ def collision_candidates(m: Union[Model, mujoco.MjModel]) -> CandidateSet:
 
   body_pairs = []
   exclude_signature = set(m.exclude_signature)
+  geom_con = m.geom_contype | m.geom_conaffinity
+  b_start = m.body_geomadr
+  b_end = b_start + m.body_geomnum
+
   for b1 in range(m.nbody):
+    if not geom_con[b_start[b1]:b_end[b1]].any():
+      continue
     for b2 in range(b1, m.nbody):
+      if not geom_con[b_start[b2]:b_end[b2]].any():
+        continue
       signature = (b1 << 16) + (b2)
       if signature in exclude_signature:
         continue
@@ -323,12 +331,12 @@ def collision_candidates(m: Union[Model, mujoco.MjModel]) -> CandidateSet:
       body_pairs.append((b1, b2))
 
   for b1, b2 in body_pairs:
-    start1 = m.body_geomadr[b1]
-    end1 = m.body_geomadr[b1] + m.body_geomnum[b1]
-    for g1 in range(start1, end1):
-      start2 = m.body_geomadr[b2]
-      end2 = m.body_geomadr[b2] + m.body_geomnum[b2]
-      for g2 in range(start2, end2):
+    for g1 in range(b_start[b1], b_end[b1]):
+      if not geom_con[g1]:
+        continue
+      for g2 in range(b_start[b2], b_end[b2]):
+        if not geom_con[g2]:
+          continue
         mask = m.geom_contype[g1] & m.geom_conaffinity[g2]
         mask |= m.geom_contype[g2] & m.geom_conaffinity[g1]
         if mask != 0:
diff --git a/mjx/mujoco/mjx/_src/constraint.py b/mjx/mujoco/mjx/_src/constraint.py
index 50efcad3..85df7e18 100644
--- a/mjx/mujoco/mjx/_src/constraint.py
+++ b/mjx/mujoco/mjx/_src/constraint.py
@@ -315,7 +315,7 @@ def _instantiate_contact(m: Model, d: Data) -> Optional[_Efc]:
 
 
 def count_constraints(
-    m: Union[Model, mujoco.MjModel]
+    m: Union[Model, mujoco.MjModel], d: Optional[Data] = None
 ) -> Tuple[int, int, int, int]:
   """Returns equality, friction, limit, and contact constraint counts."""
   if m.opt.disableflags & DisableBit.CONSTRAINT:
@@ -336,7 +336,10 @@ def count_constraints(
   else:
     nl = int(m.jnt_limited.sum())
 
-  nc = collision_driver.ncon(m) * 4
+  if d is None:
+    nc = collision_driver.ncon(m) * 4
+  else:
+    nc = d.efc_J.shape[-2] - ne - nf - nl
 
   return ne, nf, nl, nc
 
diff --git a/mjx/mujoco/mjx/_src/io.py b/mjx/mujoco/mjx/_src/io.py
index b815c32c..2f979d71 100644
--- a/mjx/mujoco/mjx/_src/io.py
+++ b/mjx/mujoco/mjx/_src/io.py
@@ -264,7 +264,7 @@ def get_data_into(
   d = jax.device_get(d)
 
   batch_size = d.qpos.shape[0] if batched else 1
-  ne, nf, nl, nc = constraint.count_constraints(m)
+  ne, nf, nl, nc = constraint.count_constraints(m, d)
   efc_type = np.array([
       mujoco.mjtConstraint.mjCNSTR_EQUALITY,
       mujoco.mjtConstraint.mjCNSTR_FRICTION_DOF,
@@ -288,7 +288,8 @@ def get_data_into(
     efc_con = efc_type == mujoco.mjtConstraint.mjCNSTR_CONTACT_PYRAMIDAL
     nefc, nc = int(efc_active.sum()), int((efc_active & efc_con).sum())
     result_i.nnzJ = nefc * m.nv
-    mujoco._functions._realloc_con_efc(result_i, ncon=ncon, nefc=nefc)  # pylint: disable=protected-access
+    if ncon != result_i.ncon or nefc != result_i.nefc:
+      mujoco._functions._realloc_con_efc(result_i, ncon=ncon, nefc=nefc)  # pylint: disable=protected-access
     result_i.efc_J_rownnz[:] = np.repeat(m.nv, nefc)
     result_i.efc_J_rowadr[:] = np.arange(0, nefc * m.nv, m.nv)
     result_i.efc_J_colind[:] = np.tile(np.arange(m.nv), nefc)