Many Dell laptops are able to use 65, 90, and 130w power supplies, but they will adjust performance accordingly. Dell Support article 12174 (KB 168345) notes:
The Dell Universal Auto/Air Laptop Adapter is a 65-watt power adapter. Dell recommends that you use a 90-watt adapter with your portable system. Using a 65-watt power adapter will not harm your system, but will cause slower performance.
Specific performance throttling will vary depending on your CPU, chipset, and GPU, but overall every component will be slowed down to afford enough power to charge the battery and operate simultaneously. Laptops that require more than 65w minimum (Precision workstation class laptops, for example) will simply refuse to charge when a 65w adapter is plugged in.