diff --git a/tests/test_to_dataset_perf.py b/tests/test_to_dataset_perf.py
new file mode 100644
index 0000000..4e46d2e
--- /dev/null
+++ b/tests/test_to_dataset_perf.py
@@ -0,0 +1,115 @@
+"""Peak-memory tests for the lazy SQL -> xarray round-trip.
+
+Asserts the lazy backend honors its contract: a single-chunk access
+peaks far below an eager whole-grid materialization, and a streaming
+aggregation does not balloon past the source size.
+"""
+
+import gc
+import tracemalloc
+
+import numpy as np
+import pytest
+import xarray as xr
+
+from xarray_sql import XarrayContext
+
+
+def _peak_mb(fn):
+    """Return ``(result, peak_memory_mb)`` for a single call to ``fn``."""
+    gc.collect()
+    tracemalloc.start()
+    tracemalloc.reset_peak()
+    out = fn()
+    _, peak = tracemalloc.get_traced_memory()
+    tracemalloc.stop()
+    return out, peak / 1e6
+
+
+@pytest.fixture(scope="module")
+def air_source():
+    """NCEP ``air_temperature`` chunked along time, ~31 MB dense.
+
+    Module-scoped so the pooch download (~3.5 MB) is amortized across
+    tests in this file. Skips when the tutorial dataset is unreachable.
+    """
+    try:
+        return xr.tutorial.open_dataset("air_temperature").chunk({"time": 24})
+    except (OSError, ValueError, ImportError) as e:
+        pytest.skip(f"air_temperature tutorial dataset unavailable: {e}")
+
+
+def test_lazy_chunk_peak_memory_is_bounded(air_source):
+    """``.sel(time=t0).load()`` materializes only one chunk, not the cube.
+
+    Reference observation: lazy chunk peak is ~1.8 MB on a 31 MB
+    dense source. A regression that quietly buffers the whole result
+    would push past 10 MB. Eager ``to_dataset(chunks=None)`` is
+    measured too as a sanity floor for the gap: the eager path should
+    be at least an order of magnitude heavier than the lazy chunk,
+    otherwise the lazy path isn't actually lazy.
+    """
+    ctx = XarrayContext()
+    ctx.from_dataset("air", air_source, chunks={"time": 24})
+    t0 = air_source["time"].values[0]
+
+    out = ctx.sql('SELECT * FROM "air"').to_dataset()
+    chunk, chunk_peak = _peak_mb(lambda: out["air"].sel(time=t0).load())
+    assert chunk.size == air_source.sizes["lat"] * air_source.sizes["lon"]
+    assert chunk_peak < 10.0, (
+        f"lazy single-chunk access should stay under 10 MB on "
+        f"air_temperature, got {chunk_peak:.2f} MB"
+    )
+
+    _, eager_peak = _peak_mb(
+        lambda: ctx.sql('SELECT * FROM "air"').to_dataset(chunks=None)
+    )
+    assert eager_peak > 50.0, (
+        f"eager whole-grid materialization should peak above 50 MB; "
+        f"if it doesn't, the eager path may have silently gone lazy "
+        f"and the lazy assertion above no longer means anything. "
+        f"Got {eager_peak:.1f} MB"
+    )
+    assert eager_peak / max(chunk_peak, 0.1) > 10.0, (
+        f"eager peak should be at least 10x the lazy chunk peak; "
+        f"if it isn't, the lazy path isn't actually lazy. "
+        f"Got eager={eager_peak:.1f} MB, lazy={chunk_peak:.2f} MB"
+    )
+
+
+def test_streaming_aggregation_does_not_explode(air_source):
+    """A ``GROUP BY`` reducing the long axis streams in a single pass.
+
+    Reduces 3.86M rows of ``air_temperature`` to ~1.3K group cells. The
+    aggregation must stream the source once and emit a tiny result, not
+    buffer the entire row set into memory. Reference observation: peak
+    ~54 MB on a 31 MB dense source (within 2x, well below a "buffer
+    the whole thing twice" failure mode). Threshold is 4x source size
+    so transient pandas / DataFusion buffers fit.
+    """
+    ctx = XarrayContext()
+    ctx.from_dataset("air", air_source, chunks={"time": 24})
+    source_mb = air_source.nbytes / 1e6
+
+    agg, agg_peak = _peak_mb(
+        lambda: ctx.sql(
+            'SELECT lat, lon, AVG(air) AS air_avg FROM "air" GROUP BY lat, lon'
+        ).to_dataset(dims=["lat", "lon"])
+    )
+    assert agg.sizes["lat"] * agg.sizes["lon"] == (
+        air_source.sizes["lat"] * air_source.sizes["lon"]
+    )
+    assert agg_peak < 4 * source_mb, (
+        f"GROUP BY reduction should not balloon past 4x source size; "
+        f"got peak={agg_peak:.1f} MB on a {source_mb:.1f} MB source"
+    )
+
+    # Sanity: values agree with the xarray-native reduction.
+    ref = (
+        air_source.compute()
+        .mean(dim="time")["air"]
+        .sortby(["lat", "lon"])
+        .values
+    )
+    got = agg.sortby(["lat", "lon"])["air_avg"].values
+    np.testing.assert_allclose(got, ref, rtol=1e-5)