When i work with long videos, final cut slows

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You have cleanmymac installed. This is bad, so bad that it has earned the nickname brickmymac.

Completely uninstall.

You have chrome and its background “keystone” tools. These run all the time and hog the system videotoolbox. Completely uninstall - see https://chromeisbad.com

Three hours is a bit excessive for videos . . . even the biggest Hollywood blockbusters are rarely that long.

It might be better to break your videos into segments or "chapters" of less than an hour and join them all together when you have finished editing.

Plugins are not installed in the FCP app so whenever you delete it there is no effect on either your plugins or projects.

There is no guarantee that removing all traces of Chrome and CMM will cure your problems but you are better without them.

Splitting your project into smaller parts is probably the most useful thing you can do.

In addition to chrome and cleanmymac, you have multiple remnants of avast. All that this does is slow down your mac and offer no security whatsoever.

Ian's advice to break things into smaller pieces is good, but even with a large 3h project your very powerful mac should be able to handle things, if not hampered by these useless and harmful processes hurting performance so severely.

Let me see if I understood: you receive a long video, and use some segments of that video in your project.

If you select a portion from the source video that comes from later in that source video, it lags in the timeline - is that it?

Does that same portion play normally if you open that same source video in Quicktime Player?

You could try transcoding the video, and see if that helps.

I would not recommend optimizing, as it would create huge files, for such a long duration.

You could try exporting just the section that is causing problems, and then import that; see if using that video instead of the original makes any difference. I think that exporting as H264 should work fine, but if you are concerned with possible loss of quality, you could export as ProRes. It would be a larger file, but since it is just a few minutes and not the whole duration, that may not be so bad.

Ian R. Brown wrote:

I still think splitting the task may help.

Create 3 projects or even libraries and edit an hour in each finally combining them in a fourth.

Ian, if I understood correctly, the OP is not trying to do a 3 hour project. As clarified in later posts, there are long video sources that he is editing into a short (±15-minutes) project. The problem seem to be that parts of these long source videos lag when added to the timeline.

After removing these problematic pieces of software, you should delete FCP and download a fresh copy.

If plugins are properly installed in the Motion Templates folder or a system library they will not be lost.

As Ian said.