slow performance in PST Migration

i ma using enterprise Vult version Symantec_Enterprise_Vault_10_0_4_R1 instaled on windows server 2008 64 bit 

i am use the PST client Driven Migration .. the PST file Stay in Queue long time(PST Submitiion in outlook client ) and when the migration start it take long time to complete i try to migrate PST file size 250 MB it take about 3 hour and 48% only migrated  .. 
how can i resolve this problem because the the pst size in my environment at least over than 10 GB