One would expect that, in line with the resultion of MC-139273 and MC-147061, that the breaking would spread outwards from the point of breakage, however this is not the case and all of the blocks break instantaneously instead. Since the nether portal is two-dimensional, this can result in sound flanging and lag for very large portals, as well as maxing out the amount of sounds that can be played at a time. I've even been able to cause server crashes due to the amount of portal blocks that break, especially when using /fill, but this could very likely be done just as well by breaking enough sufficiently large nether portals in quick succession.

To test, punch out a nether portal block in creative or displace one using a water bucket.