As Lupus said, Stat boosts remain. However, this means only the boosts like Swords Dance which actually take you to +2 will remain. Boosts like Chlorophyll which are caused by abilities and will only function as long as the Pokemon maintains that ability
So basically, Mega Venusaur will lose the speed boost.
However, do remember due to the mechanics of a Mega Evolution your previous speed is used to calculate who moves first on the turn you mega evolve. So the Chlorophyll boost will be taken into account on the turn Venusaur mega evolves but after that, dissappears.