Well, yes - but you could say the same for Outlook as well in terms of "once it starts up and stops churning, it's ok", or almost any app.
Of course after it's shown you the WLM logo, then shows your inbox, then responds for a second, then stops responding as it grinds the disk and downloads mail - these are all very simple, quick tasks that should not reduce a 4GB dual core system to a single-tasking machine while they go on. Or hell, even an Atom netbook.
I don't keep my mail client open all the time, so it's frequently launching when I click hyperlinks and such. After its cached and recently launched, its fine - but leave it alone for a few hours or just after bootup/hibernation, it's brutal. Applications much, much larger and more complex can launch quicker.
Really hope performance improvements are on tap for Live 2012/13.
Same symptoms more or less. Once I've been using it for a few minutes it seems to have "warmed up" and be running OK but the first time I perform an operation (change folder, create a new message, use the search box etc.) it seems to have to think for ages. Also, I keep it always on and if it's been minimised to the system tray for a while it crawls again for the first few minutes. I'm not exactly running the fastest system in the world, but it's capable enough. This is a mail client we're talking about, not flight sim.
I do wish there were better ways to interact with the WL developers though - I'm unconvinced that the send feedback links ever go anywhere useful, they're not on connect and generally there's no way to send bug reports or anything. They used to be so much better at this. If this was Windows or Office there would be a sense that Microsoft was excited about the new release - I'm really begining to get the impression that Microsoft is getting bored of Windows Live...