Welcome to the Wii U.

If you're someone who is rather finicky with trying to get certain jumps right, and you change them constantly, moving them pixel by pixel, I can see such loading times being rather annoying.

More importantly, can you start the levels from certain points, or do you always have to start from the beginning?

Level designers like these are by their very nature rather finicky -- especially with platformers where things have to be perfect -- and the time you spend waiting to test certain elements of your level does add up.

First world problem

Not a issue. I am sure that a performance patch after launch will improve load times.

It's short enough to avoid flaw territory, but long enough to be an annoyance.
After viewing the deaths in the feed, my index finger was instinctively twitching. The buffering time really stands out to me but that's because I've played a lot of SMB & Lost Levels to the point where I'm used to restarting right away. Although, that may be a feeling exclusive to retro platformers (or 80's games in general). I'm okay with the restart speed in say Bloodborne (post-release patch, it was annoying at launch), but I don't feel that patient with other genres. I do have different expectations and mindsets going into games or genres, I suppose this is one specific to platformers.

The game will be fine, more than fine actually...

This should be taught in all game maker schools. 

Lego City Undercover had some lengthy loading times but instead of distracting the viewer with some nice screenshoots, or miiverse posts, they put a loading bar in the middle of the screen. This only emphasized the slowness.

"But in a time when games having basicly no loading times"

What time are you living in? I'm jealous :p

Srsly though, I can think of few games that don't have load times. 7-10 seconds is not at all bad,

That's reeeeaally nitpicking if it bothers people that much for a mere 10 second load time.

In a time when games have literally no loading times? I dunno which world you're in but that doesn't sound right. Infact you frequently here of games with long load times these days...

10 seconds is about the same as Yoshi and literally no problem at all.

 6-7 seconds is about 2 seconds more than normal for a Mario game when you die, here there is not a death screen so it seems longer than it is.

In a time with no loading times?? What?? Are you living in 1992?

For what I know Bloodborne was almost 40 seconds of load screens everytime you die when it was released, too much for a being in "a time with basically no loading times"