I've been facing this issue since El Capitan. Basically, when I resize a window manually, it lags or whatever you want to call it. Here's a video in real time showing this problem.
A gif slowed down 50% can show the mouse pointer leading the entire window redraw as it calculates the layout and then resizes the window with live layout changes.
Notice how the mouse pointer leads and the window lags when you resize the window fast.
Processor: 2.5 GHz Intel Core i5
Memory: 16 GB 1600 MHz
Graphics: Intel HD Graphics 4000 - 1536 MB
Disk: 250GB SSD
CPU is not constrained when the lag is present:
Although this doesn't occur on Windows 10 (Same specs), it is still really annoying to deal with, and I'd really like to know if there's anything I could do or change to get rid of this issue...
The question: Is there a way to remove the lag when resizing a window quickly?