iMac wireless speed drops precipitously after extended time away
From your analysis you have a typical interference problem within channel 6 and 802.11b use. This interference problem might have appeared after a move of only a few centimeters. The day before you were protected from your neighbour interference with a glass, this day when moving out of the shadow of this glass you are under the full light from the AP (or iPhone) of your neighbour.

If you want to investigate this problem and really choose the best wireless channel in the exact position you are using your iMac at the period of the day you are using it there is one uniq tool to know and use:

iStumbler

Please note: the 802.11b channels [1 ; 13] are a field of interferences for Bluetooth equipments and leaking microwave owens (they are both on the
ISM band of radio frequencies).