diff --git a/auto_editor/make_layers.py b/auto_editor/make_layers.py
index 329b84b37..a0b229021 100644
--- a/auto_editor/make_layers.py
+++ b/auto_editor/make_layers.py
@@ -13,6 +13,7 @@
 from auto_editor.timeline import ASpace, TlAudio, TlVideo, VSpace, v1, v3
 from auto_editor.utils.func import mut_margin
 from auto_editor.utils.types import Args, CoerceError, time
+from math import ceil
 
 if TYPE_CHECKING:
     from numpy.typing import NDArray
@@ -234,16 +235,27 @@ def echunk(
     clips: list[Clip] = []
     i = 0
     start = 0
+
+    last_i = 0
+
     for chunk in echunk(speed_index, src_index):
         if chunk[3] != 99999:
-            dur = round((chunk[2] - chunk[1]) / chunk[3])
+            dur = int((chunk[2] - chunk[1]) / chunk[3])
             if dur == 0:
                 continue
 
-            offset = int(chunk[1] / chunk[3])
+            offset = ceil(chunk[1] / chunk[3])
+
+            if len(sources) == 1:
+                # Try to detect non-monotonically tl's early
+                max_end = start + dur - 1
+                this_i = round((offset + max_end - start) * chunk[3])
+                if this_i < last_i:
+                    raise ValueError("not monotonic", sources, this_i, last_i)
+                last_i = this_i
+
+            clips.append(Clip(start, dur, offset, chunk[3], chunk[0]))
 
-            if not (clips and clips[-1].start == round(start)):
-                clips.append(Clip(start, dur, offset, chunk[3], chunk[0]))
             start += dur
             i += 1
 
@@ -281,4 +293,25 @@ def chunkify(arr: NDArray, smap: dict[int, float]) -> Chunks:
     else:
         v1_compatiable = None
 
-    return v3(inp, tb, sr, res, args.background, vtl, atl, v1_compatiable)
+    tl = v3(inp, tb, sr, res, args.background, vtl, atl, v1_compatiable)
+
+    # Assert monotonic timeline because non-monotonic timelines because it's
+    # incorrect here and causes back-seeking (perf. issue) in video rendering.
+
+    # We don't properly check monotonically for multiple sources. skip.
+    if len(sources) != 1:
+        return tl
+
+    last_i = 0
+    for index in range(tl.end):
+        for layer in tl.v:
+            for lobj in layer:
+                assert isinstance(lobj, TlVideo)
+                if index >= lobj.start and index < (lobj.start + lobj.dur):
+                    _i = round((lobj.offset + index - lobj.start) * lobj.speed)
+                    if (_i < last_i):
+                        print(_i, last_i)
+                        raise ValueError("not monotonic")
+
+                    last_i = _i
+    return tl