Old routers running outdated protocols like 802.11g
Signal strength attenuation with thick walls etc
Distance from the WAP, speed drops away every meter.
Radio interference, there's only so many free channels and your neighbours may be blaring across your frequency
Congestion, Wi-Fi is not like a cabled ethernet link, the more people using it the lower percentage of bandwidth is available
ALL OF THE ABOVE
THE FIX? Get more new access points sited and configured by those who know what they are doing.