1 - Shouldn't be significantly
2 - Yes
3 - Yes
4 - Yes
5 - Yes
Performance can be related to a lot of different things. You should ideally start right at the database level and work forward. If the database isn't good, it doesn't matter what you do with the front end it'll still be slow. Unfortunately there isn't really a nice forum answer for my map is slow.
Start at the back end with some tools like mxdperfstat, do some tuning in your sde for things like spatial grids, optimize your symbology, hide layers, etc.
Then move forward. Check the gis server stats, the performance stats, network bottlenecks.
You might find one thing, or you could find a whole lot of little things.
I'd say the cache is your main issue. enabling dynamic caching isn't really having a cache, let alone a the scales range you have specified.