UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.87 Safari/537.36

Example URL:
https://twitter.com/googlechrome/status/992155275506286592

Steps to reproduce the problem:
1. Make sure you're logged into Twitter (it doesn't repro when logged out for some reason)
2. Visit the URL above (it works on any tweet)
3. Click somewhere in the gray overlay to close the lightbox
4. Press and hold the down arrow key to start scrolling

What is the expected behavior?
Normal scrolling

What went wrong?
Scrolling is extremely slow

Does it occur on multiple sites: Yes

Is it a problem with a plugin? No 

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 67.0.3396.87  Channel: stable
OS Version: OS X 10.13.4
Flash Version: 

An interesting note that should help debug this: the slowness does *not* occur if you close the lightbox by pressing ESC (rather than clicking in the gray area).

I confirmed that this happens with no extensions running, and that Safari and Firefox do not have this issue.