You found it slow and unresponsive because compared to most hack n slash games it is slow. However it fits perfectly within the mechanics of the game. The interface isn't a disadvantage, it just a new mechanic that needs to be learnt. I spent over 130 hours playing the Original Demon's Souls and finished it 3 time getting so far as NG++ at which point the game is 63% harder than the first playthrough and by this time I had grown so accustomed to how my character moves, fights and escapes that I could speed run most levels.
The core design is for the player to accept that they will die, many times over. But, for me anyways, every death, and I have died many times and lost at times 600 000 souls or more, was my own fault. On not one could I say it was the stupid controls or whatever.
Edit: The most souls I ever lost was about 3 or so Million. At which point I was forced to stop playing for about a week to recover. I think I nearly cried.