For example, an association using no security or WEP can be reestablished very quickly. An association using WPA-PSK...
takes a little bit longer because of the 4-way key handshake that must occur after the client is associated to the access point. An association using WPA-Enterprise takes considerably longer because the AP must relay several 802.1X authentication challenge/response messages to a RADIUS server before the key handshake takes place. Exactly how many messages are involved, and the latency incurred by each message, can vary quite a bit. Some vendors implement a "fast reconnect" authentication option to reduce this latency.
After the wireless connection is established, the client still needs a valid IP address and routing table and DNS updates. A client that uses a static IP address will complete this step in very little time, while a client using DHCP for dynamic IP address assignment will take a good bit longer. If the DHCP server is local, this step will be completed faster than when DHCP relay is used to obtain an address from a remote DHCP server.
Based on your description, I suspect that your problem is not delay in association reestablishment, but rather a problem in obtaining a valid IP address and routing/DNS updates. To confirm this, try configuring your wireless connection with a static IP address, default gateway, and DNS. (Just use the values normally returned by DHCP for this experiment, making sure to avoid any IP address used by another device on the LAN.)
If the problem goes away, then you will have ruled out the wireless connection as the culprit. On the other hand, if the problem persists, then look at the wireless connection's configuration for potential re-authentication/re-key issues. For example, temporarily disable security and see if that helps. You may also want to look closely at the router's log to see whether the wireless connection is really persistent, and run a continuous ping on the client (ping --t <router's IP>) to detect when connectivity is being lost.
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