I took out a notepad and calculated which races take longer. So people can skip those races.

GT: Silverstone and Hockeinheim are the slowest but skipping Monza and Hockenheim gives better car for GTS and Silverstone and Monza are close in lap times. Silverstone is little bit more error prone.

GTS: Slowest to Races are (in order) Hockenheim, Sebring, Silverstone

PRO: Slowest to Races are (in order) Hockenheim, Sebring, Japan, Silverstone