Routing Youtube traffic over 2nd WAN link

Unfortunately there's no Application-based routing (until v5.2), you could redirect all HTTP/HTTPS traffic using Policy-based routing though. 

Keep in mind that Google uses Local CDNs to actually stream youtube videos.  So while you may be able to set up policy or static routes for connections directly to youtube itself (based on static IPs) you will also have to deal with the IP addresses to the various content providers that Google/Youtube may use to stream a youtube video from. 

If you are not familiar with CDNs this 2011 paper gives a break down now how CDN is used on Youtube.

Having said that, a google search on "speeding up youtube streaming" returns hits from 2-3 years ago that outlines one method of blocking youtube's local CDN -- which will force videos to be streamed directly from youtube/google own servers.  I have never tried this and do not know it if actually (or still) works -- you may very well end up blocking other websites that use those same content servers.

Hi! One thing you can do, is to enable IPv6 over the link you want to send youtube (Google) traffic over.

It's more of a side effect, because IPv6 records takes priority over IPv4 DNS records.

/Stefan Lindblom