Mileage has dropped from 51mph to 33mph!!

What is your question?

Recently I had a problem where ABS, Brake System and VSA lights turned on but after check at dealership found out that due to low battery bcoz of no use for a month car had electronic system failure so they reset the system but since then mileage of my car has dropped significantly from being 51 to around 33 also I am feeling sluggishness in the pickup. Any advice guys what could be causing this problem car has 8500 miles on it and I drive around 10miles a day only