Few questions
1. Are the computers with slow speed connecting through wifi or ethernet cable?
2. If they are over wifi , what type of cards g or n?
3. What is your frequency? 20mhz only or 20/40mhz?
4. How far/distance from router?
5. What type of security key is being used? Wap2 or wap, aes or tkip+aes?
For the best possible speed consider connectioning over ethernet cable. If that is not possible check what type of wifi card is in computer. N cards will provide a connections that have a higher data rate. (results will very depending on number of antenna, manufacture of card and distance form router)
The frequency is importance because some wifi card do not support 40hmz (bonded wifi channels) depending on your location to other wireless network running at 40mhz could actually solw down speed, even if it supports higher data rates.
You security settings could also have a impact on your speed. In my opinion, Wap2 with aes only, is the best choice when using n card with no consideration for legacy support.