The promise was that WPF would use hardware-accelerated libraries such as DirectWrite to put text on the screen even faster than GDI+ (using the CPU) could do. The reality turned out to be quite different: multiple layers of abstraction and just plain inefficient WPF code [1] meant that users needed powerful CPUs and GPUs just to get reasonable performance.
[1] https://faithlife.codes/blog/2019/06/improving-wpf-text-disp...