NDMP restores very slow

Hello Experts,

I have certain experience with NDMP backup and restores. But they are painfully slow. I have worked in different data centers with backup taken on various disk based (dedupe) devices like data domain DD890, falconstor, symantec appliance 5320 and tape libraries IBM 4500, overland neo series and storagetek. filer backup  from netapp series and EMX celerra etc. Backup will run at a very good speed at about 140 MB/sec. Backups run from different volumes on various filers at good speed of approx size 2TB to around 45 GB. When restore request comes for complete volume restore like 200 GB etc, restore runs at fairly high speed and completes in 5-7 hrs approx but restore will always be painfully slow for a single file of 10MB etc. 

i have faced this problem several occasions while doing restore from disk and tape. problem is not destination filer or destination volume neither network or backup LAN speed. There is no such detail related to this problem in symantec guide. So please spare sometime and explain this slow restore which takes place on filer side.