DownloadPing Delay

I’m now investigating the reason why prefetch download of small file takes time.
Looking into Relay Debug log below about download action ID=16777875, DownloadPing was issued at 23:40:07 and its download was completed at 23:40:19 ;

23:40:01 +0900 - 1 - /bfmirror/downloads/16777875/0
23:40:01 +0900 - 1 - Closing HTTP connection on 10.10.13.48
23:40:01 +0900 - 3856 - Queueing download: {aid=16777875,index=null,sha1=null,size=null,url=null}
23:40:03 +0900 - 3856 - DownloadState: download {aid=16777875,index=null,sha1=null,size=null,url=null} changed from no state to QUEUED
23:40:03 +0900 - 3856 - Starting download: {aid=16777875,index=null,sha1=null,size=null,url=null}
23:40:04 +0900 - 3856 - DownloadState: download {aid=16777875,index=null,sha1=null,size=null,url=null} changed from QUEUED to DOWNLOADING
23:40:07 +0900 - 3856 - Processing successful download: {aid=16777875,index=null,sha1=null,size=null,url=null}
23:40:07 +0900 - 3856 - DownloadState: download {aid=16777875,index=1,sha1=61935a026adf00a81afa99291cc9f461ec9cd51b,size=null,url=http%3a%2f%2flocalhost%3a52311%2fUploads%2fCS_E9999_C60%2fFI%2f00170022%2ffi_NX_00170022.cab} changed from no state to NEW
23:40:07 +0900 - 3856 - DownloadState: download {aid=16777875,index=null,sha1=null,size=null,url=null} changed from DOWNLOADING to COMPLETED
23:40:07 +0900 - 3856 - Queueing download: {aid=16777875,index=1,sha1=61935a026adf00a81afa99291cc9f461ec9cd51b,size=null,url=http%3a%2f%2flocalhost%3a52311%2fUploads%2fCS_E9999_C60%2fFI%2f00170022%2ffi_NX_00170022.cab}
23:40:07 +0900 - 3856 - DownloadState: download {aid=16777875,index=1,sha1=61935a026adf00a81afa99291cc9f461ec9cd51b,size=null,url=http%3a%2f%2flocalhost%3a52311%2fUploads%2fCS_E9999_C60%2fFI%2f00170022%2ffi_NX_00170022.cab} changed from NEW to QUEUED
23:40:07 +0900 - 3856 - Download ping: Action 16777875
23:40:19 +0900 - 3856 - Starting download: {aid=16777875,index=1,sha1=61935a026adf00a81afa99291cc9f461ec9cd51b,size=null,url=http%3a%2f%2flocalhost%3a52311%2fUploads%2fCS_E9999_C60%2fFI%2f00170022%2ffi_NX_00170022.cab}
23:40:19 +0900 - 3856 - Download is already cached on disk.
23:40:19 +0900 - 3856 - Processing successful download: {aid=16777875,index=1,sha1=61935a026adf00a81afa99291cc9f461ec9cd51b,size=null,url=http%3a%2f%2flocalhost%3a52311%2fUploads%2fCS_E9999_C60%2fFI%2f00170022%2ffi_NX_00170022.cab}
23:40:19 +0900 - 3856 - DownloadState: download {aid=16777875,index=1,sha1=61935a026adf00a81afa99291cc9f461ec9cd51b,size=null,url=http%3a%2f%2flocalhost%3a52311%2fUploads%2fCS_E9999_C60%2fFI%2f00170022%2ffi_NX_00170022.cab} changed from QUEUED to COMPLETED
23:41:02 +0900 - 1 - /bfmirror/downloads/16777875/0
23:41:02 +0900 - 1 - Closing HTTP connection on 10.10.13.48
23:41:02 +0900 - 1 - /bfmirror/downloads/16777875/0
23:41:02 +0900 - 1 - Closing HTTP connection on 10.10.13.48
23:41:02 +0900 - 1 - /bfmirror/downloads/16777875/1
23:41:02 +0900 - 1 - Closing HTTP connection on 10.10.13.48

But according to Client log below, DownloadPing was received at 23:41:01, delayed about 54 sec ;

23:40:00 DownloadsAvailable: checking for 'http://rlhe52:52311/bfmirror/downloads/16777875/0'
23:40:00 DownloadsAvailable: false (action id 16777875)
23:41:01 DownloadPing command received (ID=16777875)
23:41:01 DownloadsAvailable: checking for 'http://rlhe52:52311/bfmirror/downloads/16777875/0' 
23:41:02 DownloadsAvailable: true (action id 16777875)
23:41:02 DownloadsAvailable: checking for 'http://rlhe52:52311/bfmirror/downloads/16777875/0'
23:41:02 DownloadsAvailable: true (action id 16777875)
23:41:02 ActionLogMessage: (action 16777875) Non-Distributed - DownloadsAvailable
23:41:02 ActionLogMessage: (action 16777875) Submitting download request
23:41:02 ActionLogMessage: (action 16777875) Download url: 'http://localhost:52311/Uploads/CS_E9999_C60/FI/00170022/fi_NX_00170022.cab'
23:41:02 ActionLogMessage: (action 16777875) Non-Distributed - DownloadsAvailable
23:41:02 ActionLogMessage: (action 16777875) starting sub action

Both clocks are synchronized.
It looks like that Client is slow to process DownloadPing.
I’d like to know how can I make Client respond DownloadPing from Relay immediately.
I have hundreds of fixlet in a baseline, so this delay of each fixlet can be over one hour delay total though each download file size is very small.
Any help appreciated.