The first place to look is available ram and cpu of the machines. Without knowing your setup a minimum recommendation would be 512MB ram dedicated to the GF installation (more would be better). You can set this in the admin console (Application Server -> JVM Settings -> JVM Options ) by adding -Xmx512m and -Xms512m properties. You should also be running the latest 1.6 jdk. You can then monitor the JVM to see what is happening on startup.
The times you quote are extremely long and may indicate there are potential programming issues as well.
Without further information this is just a guess but I hope it helps.
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