First conecton to the network takes very long

Hi,
I have a simple peer-to-peer M/S network set up on SMC wireless adaptors and
router runinning at 54 Mbps. It generally works well for internet access,
file and priner sharing. However, the first time after start up that the
laptop accesses the file system on the main ccomputer it takes several long
minutes to load any file directory. Thereafter, speeds are normal. Why does
this happen? How can I fix it? Any help will be very much appreciated