Rotoscoping first time for a 1 and half minute clip

The first suggestion I would make is to split the footage layer every 5 to 10 seconds, pre-compose each of the shorter layers, move all attributes to the new comps, and trim the layer length. This will give you a dozen or more short layers to run Rotobrush on and create a master comp with an alpha matte you can render and use as a Track Matte for the original footage. When each layer has been successfully completed, pre-compose again, move all attributes, and then pre-render these comps. You can also create a mask on the original footage so that you are only having Rotobrush look at the part of the image that needs work. If you just need to put a graphic behind someone's back, there is not need to run rotobrush on the entire body. Anything you can do to reduce the number of pixels that rotobrush will have to examine will speed up the process and help you finish the project.

The reasons for that workflow are as follows:

1. Rotobrush is a resource hog, and the longer the layer is, the higher the chance of a failure or crash.
2. Running Rotobrush takes time, and Freezing the roto after it is complete is the only way to lock a layer.
3. Pre-rendering (Composition/Prerender menu) a Rotobrushed comp reduces the AEP project size, greatly reduces the chance of a crash or hangup when rendering, and gives you a much more stable workflow.

I am lucky enough to be able to work on what is now the fastest and most capable system you can buy, and I would never consider running Rotobrush on a minute and 30-second clip. A 1:30 shot at 30 frames per second is about 2700 frames. On the latest version of AE, on my machine, a 10-second 4K clip takes about 10 minutes to trim, color correct to improve edge contrast (not for final work, but specifically for rotobrush), Pre-compose again, move all attributes, run, Rotobrush, Freeze, then Pre-Render and replace the Roto as a track matte in the main comp. I'm guessing that on your machine, the same shot would take about 15 or 20 minutes. I would not even try to work on a minute and 30 shot as a single layer. It's always best to break up your comps into the minimum length and even size required to complete the effects.

Here's a short screen capture showing the basic comp layout required to change the color of a crew member's pants so it would match the rest of the crew I recently did for a project of mine. You can see the masked and Rotobrushed pre-comp that was used.

Thanks for writing in. What's the version of After Effects you're using? Also, what are your system specifications (CPU, GPU, RAM, OS version)? Roto Brush should work fine if your computer meets the minimum system specifications: https://adobe.ly/3B2pjND

Can you try using Roto Brush on a different clip? Let us know how it goes.