Let me to be more specific:
1. Slow GUI in general
Blender uses OpenGL for the GUI. It results good things and bad things.
The good: it has the same GUI on all platforms.
The bad and more important: the responsiveness of the GUI depends on your GPU.
What does it mean in practice?
When I started Blender the first time, I was not able to work with it because it lagged like hell. I was not even able to select a directory.
I had to switch it to run on my nVidia GPU (I’m using a notebook), then I was able to use the interface.
The ‘slowness’ of the GUI doesn’t just mean responsiveness; using Blender’s inner logic and way for opening/saving/exporting files gives you a permanent slowdown on Win, because it uses its own file browser, which is weaker than the ‘original’ one offered by the Win.
2. Slow undo
In my experiences you have to wait too much after hitting undo. At the first times I thought that Blender just crashed, but no: it needs long time to revert the changes.
3. Slow 3D windows (work area)
As I mentioned before, Blender uses OpenGL. According to my experiments with other apps, using OpenGL on Win instead of DirectX results slower performance all the time.
The same amount of objects in the 3D viewport you can deal fairly enough within Max slows Blender down to almost unusable, so you have to change the settings to show ‘boxes’ instead of the real objects.
Of course using a desktop machine with a fast GPU solves this, but Max still has the advantage (and yes, Max could use OpenGL, too.)
4. Slow compositing
I like the compositor in Blender; it is one of my favorites in the toolset. But it is slow, especially when it is compared to dedicated packages.
6. Slow rendering
Except of Blender Internal rendering engine which is quite outdated and offers not so much more better quality than the rendering engine of the DOS version of 3D Studio from the last century (evil and unbiased comparison, I know),the external rendering engines are slow. The Cycles engine could be really fast, but it is not ready yet.
Speed is a quite critical point in archviz; clients want fast results.
Your employer wants fast results (you get the same hourly fee, while you are waiting for a render).
You want fast results as a freelancer, too. Because you want to finish the job and start a new and the overall time of a project reduces your profit in case of a ‘per project’ pricing.
Making archviz renders is about a fragile balance between the quality and speed.
Blender could offer the quality, but weak at speed.
On the other hand the interactive usage of Cycles GPU rendering capabilities could really speed up the texturing/lighting work-flow.
And I have another great news (I wrote about it before):
Vray is available for Blender, for a cheaper price than for Max.
So if it doesn’t loss its performance in Blender, it solved on of the most important problems Blender had.
+1. It is slower on Win than on other platforms
Most of the potential users uses Windows as their main platform.
And Blender – according to the posts I read – is significantly slower on the platform which would offer the biggest number of new users (what could raise the number of its developers, too).
I think it is not so good.