I've done some spectrogram stuff in the past, and the bottleneck (eg in JUCE where you'd want to do this kind of thing) is in the path rendering. It's pretty cheap to compute a big FFT, even throwing it on separate thread if you care enough for that. Rendering an anti-aliased path that looks decent on a small display panel with the basic vector path tools however isn't great.

Not saying it can't be done, because people do it. But making something that compares to the latest/greatest and is fast isn't obvious.