The Windows 10 preview edition has been out for a while now, and its performance is important because Windows 10 will rapidly become the dominant Windows OS - because Microsoft is giving it away to Windows 7 (and higher) consumers! So, let's see where it's at right now...
On the whole, the upgrade process was seamless and automatic, with only a few prompts about wireless networks and the like. Once booted up, I found a few features not working - in the UK at least:
With the exception of MS account support, I think that all of the above features are "new" in Windows 10, and the core Windows functionality seems to have survived intact. Still, I would not recommend doing this upgrade (yet) on a machine that you'd need on a day-to-day basis. I am trying out this preview OS on a spare laptop PC, so any loss of functionality is of little concern.
On my 4GB i7 laptop, Windows 10 seems to spend a lot of time accessing the hard disk - actually creating lag in the core UI. A quick peek at Task Manager and Resource Meter reveals that this isn't due to "swapping" (when we've run out of RAM and are using the hard disk as virtual RAM), but instead seems to be just OS-related activities.
I generally run a pretty generic configuration, but can happily report that the Garmin Communicator plugin for Firefox (Strava uploads) works just fine!