It's always impossible to accurately diagnose over the internet, but areas for consideration (based on experience) are:
1) high voltage battery cooling - rear blend door, battery fan assembly, A/C cooling lines to rear
2) Bad air/fuel mixture - airflow sensors, spark plugs, fuel pump, filters, etc
A qualified technician with a good scanner should be able to accurately identify the cause of your situation. 25mph does sound about right though for good EV mode, especially for 4WD.