You don't mention which version of FileMaker Pro and Server, but I assume they are later versions to run on WS 2012 and W10? Also, is this on multiple copies of Windows or a single PC?
We run similar setups and have no problems, having migrated entirely to WS2012. First thing I'd do would be duplicate the layout and delete everything except the portal and start from there.
Best way to test it is probably to create a new layout and gradually add elements, testing as you go. If it's simply layout corruption it will test OK all the way along. If there is an element that is tripping you up then it should show up when that element is added. I'm not a Windows user but there has been some discussion about issues with Windows 10, so browse the forum and see if there's anything relevant to your situation.
Server 15 and FileMaker 15. I tested on 2 different Win 10 machines and 2 different 2012 R2 machines.
I will avoid duplicating the existing layout so that I don't carry over any potential issues that may be underlying on the base layout, to be extra sure.
I think that is what I will have to do, that is creating a new layout and adding all the elements from scratch. I won't copy/paste any elements from the old layout which is a bit of a pain as it is quite complex.
I tried deleting and re-adding elements to the current layout see if one element in particular was slowing the layout but the speed STEEPLY decreased as I added items to the portal. And it didn't matter in particular what I items I added, as I added them the speed dropped. Adding any three objects made the portal very jarring, only 3 objects made it run worse than the Mac does with around 50 objects in the portal.