Exported video is laggy

Hello there, i am having a really weird problem with shotcut. I use elementary os, latest version(5.1 Hera). My shotcut version is 20.11.28. I am not sure which version of it. I assume it is 64 bit. My computer is a really old low end laptop, i had no problem editing some short clips in 720p. My screen resolution is 1366x768, so i was trying to convert some videos into 720p. When I edit 720p videos there is no problem. When i edit some 1080p videos into 720p without changing anything but resolution the exported video is extremely laggy, it almost skips some of the frames. Vlc player tells me exported video has exactly same amount of fps with the original one. But video is significantly slow. I would really appreciate if someone can help me. Sorry if i did not give enough information.

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The first step is to get the latest version of Shotcut.

(Hera is a version of Ubuntu Linux, which I use, so you will be getting the Linux version 21.02.27)

On the Menu bar, “help” then “Upgrade” then follow the instructions.
Or go to the Shotcut Downloads Page.

Thanks for the response, i upgraded my shotcut version but it did not help. I was having this issiue for a long time. Usually i have no problem editing 1080p videos, but only some of them giving me a headache.

Through the Shotcut site or through Hera?

On at least one other OS derived from Ubuntu Linux, the distribution of "latest packages has been lacking in quality control.
So always get it directly from Shotcut.
On Help:AboutShotcut the popup should say 21.03.21 (there was a new update released last night.)

I am not one of the codec experts here; that being said, it looks to me like the process of converting from 1080p to 720p has produced videos which require much more work from the processor of your “really old low end laptop”, causing them to be laggy when played on that machine.
That seems counter-intuitive, but it happens at times. The codec software must make millions of decisions, and if many of those decisions are “don’t send the full frame as an I-frame, make a differences P-frame or B-frame” then the CPU of the player has more work to do.

I have been installed shotcut from the original source. I am using the portable version. I guess my low end pc might be the problem here. I have another pc that uses windows. I will give it a try. Thanks for the response again.

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Update: I tried editing the exact same video on my more powerful windows pc. Result did not change. I also tried to edit same video without downgrading the resolution and it was as slow as the 720p video. However after i increased the quality from 55% to 62% the lag was not as severe. Do you think the problem is related with quality value?

P.s: My windows pc has gtx 950m, 6th gen i7 hq and 16gb of ram.

That seems to point the finger at the difference between I-frames vs B-frames and P-frames in demands on a CPU to display in realtime.

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