We usually recommend that our researcher clients who are filing I-140 Petitions in the Extraordinary Ability Alien Category (for those individuals from India or China) or National Interest Waiver Category (for those individuals not from India or China) wait for an approval of the I-140 Petition prior to filing an I-485 Application for Adjustment of Status.
Taking this approach ensures that money spent on medical exams and filing fees is not wasted in the event the I-140 Petition is not approved. USCIS generally processes I-485 applications quickly when filed following an approved I-140 Petition. We recently received an approval for a I-485 Application filed for a researcher from Kenya who had an approved I-140 National Interest Waiver Petition.