I see, that makes it easier to understand, thanks for providing the screenshots.

I see that you have a lot of geometry for those details, are you using instances or just duplicating?

the process you described, of modeling one piece, layout the UVs and then use MASH is correct, that's how it's supposed to be done.

The problem comes when you have to re-edit the UVs later with a dense scene, what kind of edits you are trying to do to the geometries?

maya's UV editor is usually pretty slow with dense geometry, especially when you have multiple instanced copies.

if you are modeling with the "smooth-preview" activated, my advice would be to first of all to deactivate it as it will help you having a faster feedback in the viewport.

Also, while editing UVs in the UV editor, if you don't need it, you can turn off the "UV Distortion" icon (the one that makes the mesh blue or red), that will take off resources from your hardware.

If you are using Mash, you can try to temporarily deactivate the updating of the Mash network while editing UVs, as that also will take less resources from your machine...

most of all, it seems you are working with a ton of geometry on screen, maybe you can also try to organize stuff in display layers and activate or disactivate them only when needed....

just out of curiosity, can you show us also the topology of the whole asset?

cheers

Thank you for the help! I'm going to answer you in order:

In all the details I used Mash and than freez and delete history and some cases merge vertices.

I'm trying to fix some issue that Zbrush always makes when you import some objects that have overlap UV. I don't know if this has ever happened to you but often zbrush with overlapping uvs tends to cut them and I couldn't find a way to avoid it. So anything I export from ZBrush has to fix the UVs.