As one who as actually used WebOS;
Startup time from a battery pull is slow, yes, but how often do you need that? You've obviously never cold-booted a Blackberry.
Bootup time is also misleading to compared to the N900. When you get the desktop in WeBOS everything is running and ready to go. In Maemo, it still loading all the background deamons, applications and widgets for some time after the window manager is displayed.

As for slow down with multiple cards, IME it's no worse, and often better, than the N900, especially with media. Videos including YouTube are sharp and clear with none of the pixellation and stutturing you get on the Nokia, and there's no breakup with music like I often get on the N900.

Here's a good example, especially with the PDF at the end:
http://www.youtube.com/watch?v=NqmvxHGVPEg

That's where the comparison should end though as Pres and Tablets are intended for entirely different uses. If I'm out and about? Pre wins. If I'm sat at home or in the office? N900.

There are 3 different types of mobile device which can be classified as to how the UI makes you behave when you want to do most tasks whilst walking down the street.

1. Ones where you just carry on walking at the same pace,
2. Ones where you have to slow down or slightly pause,
3. Ones where you have to totally stop and concentrate..

Most hardware button phones are at 1 (e.g. E71).
Most touch screen phones are at 2 (e.g. iPhone) or 3 (vanilla WM6).
Internet tablets are firmly at 3.

In other words, it's all about the flow. That's what WebOS is designed to excel at, and which Palm tried vainly to convey in their adverts [1]. A simple intuitive UI [2] which is easy to use on the move and is hitherto unknown for a touch screen, fully multitasking smartphone.

[1] http://www.youtube.com/watch?v=_1OHlFOee2w

[2] Once you've learn't the basic gestures, of course

I can text and walk with my N900.

Except for the morrning when I'm holding my coffee cup too...

That being the case I would reccommend you get one of these

Exactly, the N900 is a two-handed device. Watch out for those lampposts!
What would it take to look up the nearest Starbucks though?

On WebOS you just type the words you need from the desktop and choose from the global search (which includes Google, Google Maps, Wkipedia and Twitter as well as Contacts).

Or how about application switching or closing? Even on Maemo you have to stop or at least pause while you hunt and peck for the (oft hidden) closure or desktop icons. The UI has interrupted your flow. On WebOS, you just flick up or sideways. No concentration needed, just done.

It's not me that's walking down the street texting on a "mobile computer" with both hands whilst juggling with hot coffee!

With a good one-handed UI (Palm's forte, after all) you can keep your eye on your surroundings and the primary task of staying alive.