If you find that that the wireless PC's on your network are running really slowly, but that all the other PC's are OK, then the reason for the slow wireless connection speed is almost certainly down to re-transmission of frames. This will be due to either a high number of collisions, or interference from another source.
It might be important to establish exactly how many PC's are slow, and where they are positioned on the LAN. You may find, for example that there are only a few problem PC's, and they are all situated at remote points of the premises ie. at the limit of the range of the Access Point. In this case, there are really only 4 options:
1. If not already installed, then fit an external antenna to the Access Point.
2. Re-position the Access Point to a location where all clients are within range.
3. Move the problem clients closer to the Access Point.
4. Install a second Access Point.
If, however, you find that the poor network performance is spread more evenly between all wireless clients, then the first thing to look at is interference. The most common sources are cordless phones, microwave ovens, and other wireless networks. The simplest change is to switch to another frequency ie. change channel. This has been found to have a dramatic improvement in performance almost immediately. (You might neeed to try a few different channels before you find the best one).
If changing channel doesn't work, then there are 2 other options to consider; both of which are aimed at reducing the number of collisions on the network - RTS/CTS & Fragmentation controls.
It is possible to configure a maximum frame-size beyond which the AP will start to use RTS/CTS. This is a method by which the clients can be "locked-out" of the network whilst another client is transmitting, and which prevents collisions due to the "hidden node" problem.
Fragmentation control is really just another way of trying to achieve the same objective. In this method, you select a maximum frame size beyond which all frames will be fragmented. The idea is that smaller frames are less susceptable to interference, and also less likely to produce collisions, so the number of re-transmissions is drastically reduced and more bandwidth becomes available. If possible, monitor the network first before applying fragmentation control; if the number of collisions is less than 5% then don't bother, as no real improvement will be noticeable.
(In fact, if the number of collisions is less than 5%, and either RTS/CTS or fragmentation IS enabled, then disabling the option might well improve the situation.)