- Old routers running outdated protocols like 802.11g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.