Always wondered this one.
Desktop vs. laptop performance
Well, to answer the letter of your question, no. Two computers with the same specifications should have the same performance regardless of how they’re physically packaged. Be it a laptop or desktop.
Ultimately, though, the reality of the situation is that desktops and laptops are very different machines and they rarely have identical specifications.
Power saving processors in laptops
Laptops tend to use processors that are more power aware, more power saving, and one of the ways that laptop CPUs use less power is occasionally by being slower.
The same may be true for RAM. The same may also definitely be true for the hard disks that are installed. Many laptops include lower power hard drives that are somewhat slower than a comparable desktop machine.
Desktops and laptops are different
So, the ultimate answer to your question is sure, absolutely. If you can find a desktop and a laptop that have identical hardware configurations, they should have identical performance.
My claim is that you will not be able to find two machines, in those two different configurations, that have identical hardware configurations – simply because the hardware chosen for each respective configuration is targeted to the typical use for that machine: desktop versus laptop.