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No, you can keep zooming out if you want to and it won't stop you. If you want the grow or fit modes, you can access them quickly via G and F on the keyboard (at least in English) as well as the toolbar buttons.
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There isn't a denoiser, at least in the sense of the word that I understand.
The viewer uses the basic resizing provided by Windows, which is usually a bilinear filter when enlarging, I believe. (In some cases, where Windows feels the image only uses a few colors in blocks, it can use nearest neighbour instead. That choice is something Windows decides.)
We may add more control over resizing algorithms in future, but the bilinear one Windows usually uses is fairly fast and should not take a large amount of time unless the images are enormous. You can view things at 100% size to avoid scaling entirely to test whether it's the scaling or something else slowing things down.
There's no lengthy image processing applied to images as they load, other than potentially for Raw Digital Camera images (but only if configured to do a full decode, or possibly for some rare camera types that don't include previews/thumbnails within their files).
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I am not sure what you mean, sorry. Rename the files in what way? Please start a new thread for that with more detail about what you want to do.
As a general note, please ask one question per thread, unless the questions are closely related: