No, neither of these are true. If Win27k startup is an 8k 120fps video, it is
either latency
or stutter if you don't have enough bandwidth. You can absolutely design a system with priorities set such that latency is above stutter-/drop-free playback, and if you do, the startup time will be unaffected by that bandwidth.
And, no, not all bandwidth is built to reduce latency. There is a lot of bulk, best-effort traffic - for example, YouTube and Netflix proactively distributing videos between datacenters across the world. (They totally do that before anyone ever clicks play, they have enough data to know what is likely to be needed where.)
The same applies to your YouTube/Netflix playback at home. It doesn't need to be low latency. The only effect of latency is a longer time between you clicking play and playback actually starting. From there onwards, you just need enough bandwidth to keep the buffer filled, and you can do that quite a bit ahead of reaching playback position. Latency is a real non-issue there.
Same locally for bulk copying files around. If your OS & FS is designed well, latency only shows up at the beginning of the operation. Most file systems were designed when data was on rotating rust, and that's dealt with readahead and the likes.