AFAIK from casual encounters to comic writing advice (I read stuff that sometimes has them) the actual workflow is more like:
Write a summary of the story, with the sequence of events you want to have in it, and the characters, props and places relevant to events described to degree sufficient for yourself.
Split the sequence of events to pages. Each page should have the same amount of content and the flow should not be too broken or uneven. (The amount of separate events per page should not be high or highly variable.) This will require more detailed descriptions of events. You might need to merge, split or otherwise modify the events to get a good flow.
Split the pages to panels and plan the panel layout. Decide what goes to each panel and plan the layout to reflect the contents. Might need to add or merge pages, which might trigger a partial rewrite...
Write the actual script. This adds detail to visual descriptions and writes actual dialogue instead of just descriptions of what is said. You might find some panels have too little content and some have too much, which might force you back to step 3, which might prompt going back to step 2 and so on...
The point is that by the time you write the script you should already know what is in the panel. So the order you use at that stage should have little impact on time needed. I'd suggest going iteratively, so that you see whether your panel split and layout are good before putting too much work in describing individual panels. Also the speech bubbles and SFX are a part of the visuals of the panel, specifically you need to know that they fit, so doing things separately might cause an issue it takes time to fix.
It is possible to work bottom up, script panels first and then plan panel layouts and page splits based on that as your question seems to assume, but as comics have a natural top down hierarchy of story pages panels it is unlikely to be the fast option. IMHO top down also makes easier to design the panel layouts, but you might not want to make that a priority anyway, just checking that the panels will fit their pages might be enough.
Again I am hardly an expert on this, but I hope this helps.