Hi, i've been developping on DNN since 3.1 and I don't think there's quite a speed issues with it.
DNN 4.5x focused on performance, and it has ajax built-in. The DNN team also has Microsoft support and does upgrade the framework every month or so, so you can be sure it's stable and secure.
The architecture does take a little bit of time to master as any other CMS, but once you master it you can do pretty cool thing quite fast. I would suggest staying off the DNN filesystem for your custom development though.
I never had the chance to use DNN on shared hosting so I don't know if it would be an issue, maybe try looking at the dotnetnuke forum for some feedback on this.
As for LLBL, never had one single problem with DNN, you just use LLBLgen as if you were using a regular application/asp.net website. Most of my DNN Module development is done using LLBLGEN and it's working flawlessly.
You can reach me on MSN Messenger at this address: mik_18_xix@hotmail.com