Environment

Windows build number: 19619
Windows Terminal version (if applicable): 1.0.1811.0

Any other software? No

Steps to reproduce

1. Use WriteConsoleOutputW function to fill console with block characters and random foreground colors.

Expected behavior

Smooth and high performance

Actual behavior

1. The new windows terminal performs very bad (same with powershell or cmd), only refreshing at a few fps. While the old terminal can easily get to a few hundred.
2. And selecting the filled area after that is very laggy in new windows terminal, but it's very smooth in old windows console. See my gifs below.