Without intending to criticize your relationship to your girlfriend, I think your experience presents a lesson in client-provider relations. It’s absolutely vital that you understand your client’s needs before running off and building them a solution they may reject out of hand.
An overpowered Windows machine is best suited for gaming, not design work. Vector graphics tools like Sketch work fine on a MacBook. They don’t need a desktop grade CPU with tons of RAM and a powerful video card.
Her aging Macbook Pro definitely struggles with the load. So much that she's asked me for help to figure out what's wrong. But a large Sketch project, like a large Photoshop project, is just resource intensive.
An overpowered machine would definitely be welcome. My point was to emphasize how a crossplatform app would let her take advantage of the opportunity (a new computer I built for one game that I'm giving away) where a native app does very little for her.
I use a Macbook myself and encountered the same thing on the new Windows machine. Every application that was Mac-only was just downside. It made me extra motivated to finished shifting my work environment to crossplat apps like VSCode. And CLI tools that worked well in WSL2.
An overpowered Windows machine is best suited for gaming, not design work. Vector graphics tools like Sketch work fine on a MacBook. They don’t need a desktop grade CPU with tons of RAM and a powerful video card.