Queuing large number of downloads slows down the process

I am trying to use the downloader plugin to manage download large number of files ~5000. I am calling enque method in a forEach loop to populate the download Queue. I see following problems
1. Adding download in queue takes time as the queue size grows > 2000
2. If there are say 900 files added in queue, Plugin shows around 500-600 files in running state and rest in "queued" state. I expect running count equal to the value of parameter "FDMaximumConcurrentTasks".

Please suggest better way to manage large download queue.