ISP core networks typically have two components, routers and dwdm/opto equipment. The routers are not really special, just your regular ISP core/edge router, adding roughly the same delay for each hop (invisible mpls hops or not) on the order of microseconds.

Opto equipment adds practically no delay as they just forward the light without looking at it or processing it. There might be redundant paths that it can switch between if there's a fiber cut.