It probably has a cheap extruded base. Extruded bases are slower, especially if it's a cheapo board. I was riding a board with an extruded base and recently switched to a higher quality sintered base. I have way more speed now.
I don't think a 140 would be the right size. 140's are for the 100 poundish people, like 100-115. At 70 pounds, she's very light. Something in the 130s would be more appropriate. However this all depends on the models weight specs too. I was riding a 141 at 115 pounds and was right in the weight category for my old board.
IT AINT EAZY BEIN STEEZY