Simple words, this was a great movie. I enjoyed it, also the video did not buffer or lag and that made the experience better, but I also paused and delayed watching for a day because I was tired.
The first 11 minutes into the movie, it already felt like a real movie or it felt like it was already done or only to me it was. It felt like it and it felt like I watched an hour, but it was actually 11 minutes. Then 45 minutes later, and I thought it was done, but maybe not.

Long version:
The story is kind of weird; so you have this tunnel that goes to nowhere, has sets and rows of trees that have metallic diamonds cubes attached to them, and looks like memory lane with autumn leaves and an endless pond or lake. It will seduce or captivate your mind, heart and desires into walking into it, and never come back. It will generate an endless loop of memories in exchange for your life; the memories seem real, but they are just an illusion, like in Spirited Away, with the spirit world that is an illusion, but is real in its own way. The tunnel eats away at your lifespan or life-force, is what I think it does, and after that, if you have been in there for 100 or more years, you will become trapped in it. And, the only way to break out or break free, might be to experience this sensation more than once, experimenting with time and communicating with the outside, then making sure to enter, but also have a strong conviction and will, so that you can come back, and if you don't, you would become trapped in illusions forever, like looking back when exiting the tunnel that leads to the Spirit realm in Spirited Away. To the tunnel, the people or animals that enter are just food or energy that is being fed to it. And, there was also the fact with the broken Torii gate outside the tunnel, it could have been a sacred shrine at one time, but was abandoned and it may have fed on peoples life-force to become alive to lure people and animals into it to feast on them. There is, and it seems so, no exit whatsoever on the other side, it seems that the only exit is the entrance and if you run too far, it would be hard to get back.
Well, that is my hypothesis of it, based on some references and the synopsis and movie scenes. And, also what I think of the tunnel. I mean, it could be some kind of youkai, if you think about it.

I kind of wish there was more to this movie though, the ending was alright, not bad. However, I wish they showed us how their relationship went, or did the male protagonist go back to school because of his age being younger and he did go missing about 13 years ago, because of having a falling out with his father and running away into the tunnel. But, more importantly, it is kind of strange, their age difference at the end of the movie and they were fine with it, unless he aged and I didn't notice.
I guess, if such a tunnel really does exist, I would for sure enter into it, because hours inside is years outside, and because it has realistic illusions, I feel it would be nice to play games inside it or relive the same day again.
I'd recommend that if you haven't watched this yet, then you should definitely watch it as I heard it has some good visuals in it or good artwork. The music or OSTs are really smooth too, they go well with the animation and it sort of sounds emotional near the final scene.

To be honest, I thought that after the male protagonist went inside the cave, I thought he would be there forever or something, or never to return to the real world. It felt like he was going to stay there, but it was unexpected that he woke up to reality. I also thought there would be some kind of betrayal somewhere in it, I thought the female protagonist would betray his trust and enter without him. And, before watching this movie, I kind of think I watched it already somewhere, but I forgot where and it feels like a Studio Ghibli movie, I also thought they would both enter and never return or something, but the plot was very unexpected. I would want to watch this again, it feels nostalgic or something.

I will mark this as a spoiler though, I kind of say too much and all of this kind of sounds gibberish if you try to read it. I dunno why I typed this up, I kind of spoke my mind.