Perhaps try your graphics settings? Lowering them may help with the lagging on the larger lots. Perhaps also your computer is not up to specs to be able to properly load larger lots. Also if it is a residential lot that you live in that is lagging, try checking their Sim inventories. High amounts of items in Sim inventories can also make lag on large lots worse. Also do take in mind that there is a general 20-Sim limit to prevent lag on all lots; you may have a problem with a mix of these.