Microsoft virtualization guru Ben Armstrong explains on his blog site why Hyper-V virrtual machines seem faster once they are up and running, as opposed to why the installation of a guest OS on Hyper-V seems to take longer than it would on Microsoft Virtual Server 2005 R2.  He explains the reason as such: 

With Virtual Server and Virtual PC we only had emulated devices to use - and as a result we spent a lot of time optimizing and tweaking the performance of these emulated devices.  When we implemented the emulated devices under Hyper-V we had to remove many of these optimizations due to the entirely different architecture of Hyper-V.  We did not, however, spend much time re-optimizing the emulated devices on Hyper-V because we had the new synthetic device architecture where we have focused our attention for performance tuning. 

This means that Hyper-V emulated devices are slower than Virtual Server / Virtual PC emulated devices - but Hyper-V synthetic devices are much faster than Virtual Server / Virtual PC emulated devices.

The catch here is that when you install an operating system you are almost always using our emulated devices - and you do not start using synthetic devices until after you have installed the operating system.

So in conclusion:

1. Yes - operating system installation on Hyper-V is slower than on Virtual Server / Virtual PC.
2. No - I do not expect this to change much for the first release of Hyper-V.
3. Yes - once you are up and running and have integration services installed performance of Hyper-V virtual machines is much better than Virtual Server / Virtual PC.

Considering there probably aren't (or there shouldn't be) many people who install one off virtual machines (actually install the guest OS inside each virtual machine, everytime), this shouldn't be a big deal up front.  I would assume that in future releases, Microsoft will "fix" this.  But since most shops create a master image and then upgrade/update/modify it to suit their needs rather than installing Windows over and over again fresh with each virtual machine, I think the slowness of installation is something they can live with in the first release.

You can read, comment or find out more on Ben Armstrong's site, here.