How much delay is normal in publication of a Special Issue? I submitted a paper to a special issue of a very reputed computer science journal. Now as per the original timelines, the special issue was expected to be published by this month with two cycles of revisions. However, there was an extension for a month in submission deadline. 

I submitted the paper within the extended deadline and yet to receive even the reviewer comments (4 months since submission while as per timeline the period for first review was two months). During this time, the manuscript was in peer-review. It went to "Awaiting Decision" three weeks ago, but I am still to hear on any decision. 

Since this is my first submission to a reputed journal, is it normal for special issues to be delayed to such an extent?
From my experience, special issues are rarely published on the announced date. The review process for papers submitted to the special issue should be as rigorous as for any other journal article and thus, may experience all the usual delays (referees not answering, promising a review but not delivering, contradicting reports…). The only difference is that the journal has announced a time frame for the whole process which puts a little more pressure on the handling editors. However, this does not necessarily imply that the whole process is any quicker (also note that the guest editors of a special issue may not be as experienced).

I do not know too much about the usual delays for papers in computer science, but for math, the times you describe still sound pretty short.