This was the first time I ever played this game (other than briefly to make sure it worked when I bought it a couple years ago) and it took me about 6-7 hours over two playing sessions to beat it.
The spike in difficulty is pretty bad. From the beginning through Stage 7-1, this game is very easy. Then 7-2 becomes pretty hard. I probably spent over 90% of my time on Stage 7-2.
Most of the actual difficulty of this game comes from the terrible hit detection. Countless times, your sword will be inside the enemy, but the game will not register it as a hit, with the bird-like enemies being the worst offenders. Also, you will be damaged by what can best be described as a very large aura surrounding the enemies (I got damaged countless times by an enemy who cleared me by at least a head when it jumped). Again, the bird-like enemies are the worst as it seems that what you see of the enemy seems to take up about half of what will actually damage you. Pixel-perfect is a term that cannot even come close to being applied to this game.
The distance you bounce when getting hit is laughable. Even more laughable is when you are running left, the enemy hits you while also moving left, and you end up bouncing half of the screen RIGHT.
