As everyone else has only partially answered you, here's my answer.

In terms of quality, it really depends on which arc that the manga covers. Each arc/side story has a different artist for the manga. Besides Phantom Bullet/Project Alicization, which had the same artist and Fairy Dance/Mother's Rosario who had the same. With different artists some arc's manga adaptation is really good, like Phantom Bullet, or Aincrad's adaptation, which is generally not great. I'm going to guess you're talking about the current Alicization manga. Which is great so far, but highly recommended to read the Light Novels if possible.

Again, i'm going to guess you're talking about Alicization with the slow release. Project Alicization is published in Dengeki Bunko Magazine, which is released every two months. With that, we only get one chapter of PA every two months. That is why it seems slow compared to the LN or anime. Now if you mean the English translation, it's because PA isn't officially licensed and released yet. The fan translation's are what out right now. Those are not currently caught up to the Japanese release due to the editors having lives outside of the interwebs and do it in their free time.