"xxxxx as I am" can be viewed as an archaic way of saying "because I am xxxxx" - so the original sentence can be rephrased as :
How slowly the time passes here, because I am encompassed by frost and snow!
(See http://www.dictionary.com/browse/as definition 8)
Note how this works for all of TRomano's examples.
Similar to the use of "as" in a sentence like "As it is Friday, I am leaving early" being rephrased as "Because it is Friday, I am leaving early"