[Bug] render slowing down significantly after frame (70x slower)

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Needs clarification
Activity status
Quiet
Tech stack
blender, python
Domain
computer-graphics

Research direction

Start by reproducing the lengthy cloth simulation and MP4 render in Blender 5.0 on Windows 11 with the RTX 4070, measuring frame times around frames 100–120. Compare normal rendering with rendering after restarting Blender, and document a reproducible cause or the evidence needed to isolate the slowdown.

Written by the indexing model from the issue text.

Description

bug worked
Summary

unsure if this is a bug or hardware limitation, but while rendering, after some amount of frames (somewhere in the vicinity of 120 frames) the render slows down by 100 times, one frame takes 10s to render, the next takes 6 minutes; when i close and reopen blender, and start rendering from where i left off, the animation continues as usual, taking 10 seconds, but after another 120 frames or so, the same problem happens, and so i have to redo this about every 120 frames.

I have fetched the frames but not baked, and there is nothing else in the scene but the simulation and basic lighting, nothing is animated except the cloth (only the cache if it counts as an animation)

Steps to Reproduce

make a lengthy simulation, and render as mp4 format, after a while the frames will jump from normal duration to about 70 times the duration

Expected Behavior

all frames taking the same amount of time to render as there is nothing else in the scene

Actual Behavior

frame render time suddenly jumping to 70x the duration after roughly 100-120 frames or so

Environment

windows 11, nvidia rtx 4070 laptop, blender 5.0, version from yesterday before the latest update

Contribution Agreement
  • I confirm that any code, patches, scripts, attachments, or other copyrightable material I include in this issue may be used, reproduced, modified, and distributed by this project under the Apache License 2.0 or as otherwise described in CONTRIBUTING.md.
  • This is a bug report from the community (please provide a reference below).
Community Reference

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Dominant language
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