Depends what you mean with slower - when its not messing about with updates and stuff 10 usually performs very well some exceptions aside and generally has better low level performance than 7 i.e. if you are using a program that is heavily threaded.
However there are quite a few areas like explorer where 10 can get very bogged down and run much slower than 7.
Overall the UI is a horrific mess - no idea how anyone can defend it unless they are so young and/or so new to PCs that Windows 8 is a large formative part of their OS experience - stuff is quite fragmented or unnecessarily obscure and just looks nasty - having now seen 10 on a Windows phone (where it does actually look good) I can understand what they were going for but on a desktop it simply lacks the aesthetically polished look of 7.
Some things are better or have the potential to be better - despite the lack of flexibility, group management options, etc. once you enable the extra tile on the start menu and strip it back and rebuild to your preferences it is a powerful tool and far better than 7's start menu but unfortunately let down by the lack of polish in the overall implementation.
EDIT: In terms of navigation to find stuff its very hit and miss (partly due to the mix of options, etc.) some stuff is very quick to get to/find other stuff is hidden away and can take longer to work with than 7.
