What is ids11 new function RSS for?

Hi ,
I had read informix ids version 11 administrator guide about
RSS (remote standalone server) ?
I am wondering about page number 434 :
" The high level of coordination between a primary and secondary HDR
pair can cause performance issues if the secondary server is located
on a WAN (Wide-Area Network)."

So, It mean :
HDR not suited for WAN ,
and RSS is suited for WAN.

Why ? what make RSS suit for WAN?

.