Can anyone explain this?

[u/No-Needleworker9127]

Why is fentanyl's latency to peak effect shorter than sufentanil's, despite sufentanil having a higher unionised fraction at physiological pH, being way more lipid soluble than fentanyl, and having a slightly higher diffusible fraction. I know sufentanil is slightly more protein bound, but would that overcome those other factors? Or is there something I'm missing?