Mine is hanging on the Cloud Load screen for as long as a minute. I've not Flashed, Rooted, Loaded an .apk or any other config. All the applications I have are directly from the Market.
just to make sure im understanding this,you have a stock,unrooted,unmolested phone,correct?
I just got back from Verizon and we compared it to a floor model and they both seem to hang on the cloud screen. I don' t recall it hanging that long. The boot cycle seemed a lot faster when I first got it. What prompted me to even reboot the phone was the fact that Slacker Radio as of yesterday is stalling and hanging.
There isn't any way to completely remove Slacker and reload it. To answer your question, it would completely boot, it just seemed a lot slower than when I first got it a week ago.
have you checked to see if slacker is available in the market? if its not,i could email you a .apk of slacker,but all the ways of deleting it will require you to be rooted.
if i sent it to you,im not sure what would happen when you tried to install it with the old one still in system/app. prolly one of these 3 things:
1)it will fail. tell you its allready installed,and wont let you overwrite a system app.
2)it will ask you if you want to overwrite the old one. this could work out for you if thats how it goes. or it could eff it all up... i dont really know.

3)it will install it with the other user installed apps in data/app,and youll have 2 in your app drawer with no way to tell wich is wich.
so i guess you have a couple options:
-take the phone back since a system app is not working correctluy
-shoot me an email addy youd like the .apk sent to if you want to try installing it.
- root your phone so you are able to delete system apps

-try pandora radio and see how it works for you. its similar to slacker.