When i export a project who is a part of a lake with a boundary for the full lake into an at5, the time to process is really longer i think IMC export all there is in the boundary...

Thanks you

It's probably fair to say that it's neither a bug or a "feature". It is by design, but it does seem that a lot of people are using shorelines much larger than their maps, and for exporting raster this can make things very slow.
We'll probably add an option (or just change the default behaviour) to include just a clipped shoreline in the same way that the contour view shows. Having said that, being able to export a shoreline without a map can also be useful. Note that if you export a vector map then the extra shoreline doesn't materially increase the time taken.

Thanks for ur answer, so the solution for now is to export in vector ?

It's up to you - vector is much faster to generate, and results in much smaller map files. Raster can be nice for shaded relief. Alternatively you could just use a smaller shoreline - i.e. clip it to more closely match your mapped area, so you are not generating a larger raster area than you have to.