r179397 added Parallel.h and implemented it terms of concrt in 2013.
In 2015, a cross-platform implementation of the functions has appeared and is in use everywhere but on Windows (r232419).
r246219 hints that <thread> had issues in MSVC2013, but r296906 suggests they've been fixed now that we require 2015+.
So remove the concrt code. It's less code, and it sounds like concrt has conceptual and performance issues, see PR41198.
I built blink_core.dll in a debug component build with full symbols and in a release component build without any symbols.
I couldn't measure a performance difference for linking blink_core.dll before and after this patch.
(Raw data: https://gist.github.com/nico/d4b02c7dd835bb96ed67e919f3558e6f)