A particular combination of TIFF formatting options causes slowness

[ddohler]

Tiffs compressed using LZW can take several orders of magnitude longer to decode than those encoded with Deflate. Using native code, there is little difference. This is probably an Emscripten limitation or artifact.
This actually appears to be a very specific combination of factors that causes a huge spike in time to thumbnail a tiff. The known factors so far are:

• Tiled TIFF layout (switching to striped resolves the problem)
• Large size (there is a huge spike in time required between a file with 59% of the original file size and 60% of the original size)
• Possibly also Byte pixel values and multiple bands, but this is untested.