The performance for accesses to shared folders from a Windows guest might be decreased due to delays during the resolution of the shared folders name service. To fix these delays, add the following entries to the file \windows\system32\drivers\etc\lmhosts of the Windows guest:

255.255.255.255        VBOXSVR #PRE
255.255.255.255        VBOXSRV #PRE

After doing this change, a reboot of the guest is required.