I'm having similar slow issues with uploading to ESXi 5.0 on one of my customer's IBM server, they have two identicle servers, both have IBM direct attached storage, one of them uploads fast the other one is slow.
Transfering files to a Windows VM located on the slow server is fast for the first 5 or 10GB, from what I have read is that Windows stores or caches the writes to memory first then writes them to disk, when I start the transfer I can see the memory usage on the target windows VM going up from 2GB to 6GB, as soon as the transfer completes the memory usage drops again, the first few GB of the transfer goes at around 50MB/s then it slows down to around 3MB/s.
Copying files on that storage from one folder to another via the vi client browser is also slow, downloading the files from the storage back to windows is fast.
With the other server transferring files to and from the storage is fast regardless of what I use, vi client, Veeam, windows VM.
The slow server also has another direct attached unit connected to it via a different host adapter, uploading to that storage is fast.
At this stage I believe that either the write cache isn't enabled on the Raid controller that is connected to the IBM storage or that it's set to write through, I'll do some testing in a few days and let you know the outcome, it has an IBM MR10M controller with battery backup.