
Is "lince" slang? I've never heard of calling someone a lynx, so to me it sounds non-literal.

"eres un lince" means "you're really clever", I don't know if it's slang, although I don't think so because it appears in the RAE with this definition:
Lince:
3. Persona aguda, sagaz.
http://buscon.rae.es/draeI/Srvl...S=3&LEMA=lince