Hi there!  I do highly doubt that any plane will fly slower at night then at day.  With most things in aviation, going slower then your supposed to usually leads to a safety concern, more importantly, stalls.  A modern day 737 typically rely mainly on its instruments (and autopilot in a lot of cases) to fly, especially at night time.  There would be no need to slow down, and even by doing so, it would have negligable results.  It may be looking slow from the angle your looking at it.  I know at times when im driving, and I look up at an airliner on final, it looks like its just hovering.  However, if angle is not the case, then the only explanation could be perhaps Air Traffic Control spacing, which I doubt, but maybe...  

And just another note too, depending on the loading of the airplane, its approach speed will differ.  It can be as slow as a cessna 172 on a fast cruise speed sometimes as well.

Maybe he had a strong head-wind.

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I dunno-I have something like 4.5 hours of night flying around the Philadelphia area (1.5 x-country) and I always have trouble finding the airports, every time.  I can spot them much easier in the daytime.

But thats just me.