I often find myself in the wrong screen mode (landscape vs portrait) and I find the switching delay quite annoying. I would like my phone to be much more responsive when I rotate it (iphone-like)... Do you know about any apps/tweakings to achieve this?
I have searched for the same thing, and unfortunately, have not come up with much of anything. What I have settled for is turning off auto-rotate, and only using it for apps where I really need rotation. I do recall there being an app which allows setting auto-rotate on and off on a per-app basis. I believe it is called Smart Auto-Rotate.
Part of what jlehenbauer wrote is correct in that ADW only gives you an option to have auto rotation or to keep your home screen in portrait or landscape mode. Depending on how many home screens and what widgets and types of widgets, folders etc you have on your homescreen it will determine the amount of memory your home screen requires and how long it takes to refresh it when it rotates between landscape and portrait and thus how quick it does it. The iPhone doesn't have widgets (the main cause) and thus it is quicker at doing this. Due to some widgets HTC Sense actually locks the home screen in portrait mode and doesn't allow you to change it.
Currently I am using Go Laucher where i have experienced what you say - the home screen refreshing on orientation, however as stated you can speed this up by removing widgets, especially any scrollable ones. Try setting up one of your home screens as blank, and then see how much faster it will switch between the two states. The only time I have sensitivity options and more than just portrait, landscape or auto for the home screen is on a cyanogen rom. However you would be looking at rooting. I haven't checked out every launcher app available, so its possible you might find a better alternative.
In regards to app killers - no longer necessary unless you have a rogue app (in which case would seriously suggest removing it full stop), all they do is drain your battery.
Really the only things you could do is lock the rotation of the screen from the "Settings/Display" menu, so you wouldn't have to deal with rotation, or start freeing up space on your device.
This process will be taken care of by the available memory, so if you have a lot of things running, it will be sluggish.
The iPhone was speedy at this previously because it didn't have the ability to run things in the background, and since gaining this ability has found a similar stumble. A good appkiller will help, and i find deleting old, unused apps every now and then helps system speed.