Preview starts to lag after render

[linaki-org]

Describe the bug
I am encountering laggy preview problems in the sequencer on the latest 2026.11 version of Xlights. I investigated a little, and the strange thing is that, when I load the sequence, it previews perfectly smooth, no problem, but as soon as I render my sequence, the preview starts to feel laggy. I don't think it is a problem with rendering a lot of pixels, because if I save after that render and then reopen, everything is smooth again.

To Reproduce
I don't think it is related, as I had no problems with my previous sequences on the same setup, but I am running the sequencer on an external 32" screen, so maybe it causes performances issues...
As I said, all my previously made sequences still work fine even after rendering, I also tried to add complex effects or whatever but still nothing.
I attached the packaged sequence so you can try to reproduce the issue.

Expected behavior
I expected the preview to act normally, by rendering the sequence smoothly like it already did, even after rendering.

Screenshots
I attached a video of the issue in the attachments below

Versions (please complete the following information):

• OS: Windows 11
• xLights 2026.11 (latest)

Additional context
Add any other context about the problem here.

Attachments

• My packaged sequence : Here
• Video of the bug : Here

[derwin12]

Not duplicating here .. A bit hard to see on your video. I even put the house preview to my 60" tv. Might need to look for updated drivers or perhaps check task manager to see what is using up cycles. Try turning off / on hw acceleration preference.

[derwin12]

If you feel it is just this sequence .. make a copy and go to model -> delete effects and go thru the sequence to see if there is one particular model causing the issue.

[linaki-org]

Yes I looked in the tasks manager and XLights uses only 10% CPU, 900Mb of RAM before render and 1.1Gb after rendering (btw is it normal it uses so much RAM ?). The very strange thing is that it works perfectly fine before I render. I don't know how the preview engine works, but I imagine it plays the render, so why would it be different before vs after rendering ? I'll try to update my drivers, and I'll tell you

[derwin12]

I believe it holds the fseq in memory for the playback...so yes it could grow a bit. If you have a bigggg matrix that will consume lots of memory (you dont). Is the .fseq on slow disk? Really guessing here..

[derwin12]

You are on windows .. so you can simply rollback to .10 and see if it helps. That can perhaps narrow things down.

[linaki-org]

Yes, I do have a 10000 pixels matrix, but I have a Nvme SSD so don't think the disk is the problem. My drivers are already up to date. It seems like if I render, wait a bit and come back later it comes back to normal. I tried downgrading to 2026.10 but still the same issue. It is not a huge problem, I can totally work with it but I wondered if there was anything to do to fix that.

[linaki-org]

The issue is mostly seen on the fast single strands chases, they are a little choppy after rendering.

[derwin12]

I would try disable render on your big matrix and see if that is the issue. It could still be rendering or something in the background? or just outputing those channels. Not sure.

[linaki-org]

Thanks for help, I tried a lot of things and none of them seems to work. In fact when I asked the question, I was thinking this may be a bug related to the way sequences are rendered, like a memory problem or anything. I'll continue digging to try to understand what exactly causes this lag.

[AGFazio]

I tried your sequence on my machine (Win 11) and no issues. Rendered and played fine.

[derwin12]

Perhaps drop into the zoom room to demo this. As of now we have no where to go with this.

[linaki-org]

I tried that same sequence again today just on my laptop, and it seems like the problem was simply the big second monitor, even if the preview was on the main laptop screen, with xLights using less RAM (now at 650 MB vs 970 with the monitor). Even if other sequences worked fine on this same monitor, maybe as this one is a little more complex, it causes issues with managing the sequencer display on the 32" screen. Thanks for help !