Code running slow after upgrade.

Hey guys im jsut looking for a few opinions on this, we recently updraded our production code from vb.net to c#. The old way returned datatables which we binded to asp grids. The new way returns

List<GridObj>

and we then bind to the grids. The problem is the new code seems to be running a lot slower than the old code but I have no idea why. I'm thinking that maybe the indexes on the sql tables need to be rebuilt but I think anyway this would only be a temporary solution. If anyone can shed any light on the situation I would be very greatfull.