No, GlassWire does not route traffic through itself. It uses a Windows API.
GlassWire uses different resources depending on how it's used. If you're using something like Bittorrent that accesses lots of hosts simultaneously then GlassWire has to work hard to keep up.
We are working on an optional version of GlassWire that uses significantly less resources for people who use apps and services like Bittorrent.