Does the 15" and the addition of extra cores and a GPU make a big difference? I'm using an external monitor as well as the in built one, I've got various apps open such as Chrome, Royal TSX, Insomnia, Outlook, Excel, Apple Mail, S4B, Atom, VMware Fusion (one VM), about 30-40 browser tabs and Tunnelblock for my VPN.
Animations and switching between screens especially with VMware Fusion running Win 10 I get a lot of slow down in terms of animation and the speed of the VM is pretty crap (got Powershell ISE open only). If I run Powershell ISE in Unity it's even worse. Current Mac is a 2017 13" with a dual core i7 and 16GB of RAM. Don't get me wrong the machine overall is not slow, just certain animations are as is the VM (2 vCPU/4GB RAM).
Actually I've just noticed RAM usage is north of 14GB.