usually a drobo can become slower when it fills up quite a bit (just like other devices too), though can you see any usage/free space values changing every 10 mins or so on dashboard currently? (it might still be reclaiming some free blocks for you, or optimising things depending on how many files you removed or how long ago you did that)
do you know if any other programs or processes are also possibly using/writing to the drobo (in addition to what you are doing yourself)? for example, maybe an antivirus scan or content search indexer is running too? (though that would seem to affect reads more than writes)
thanks for the info chris, (if you happen to have any more info about those widgets, they also sound interesting)
if you have not seen any space reclamation taking place in dashboard, then it could also be a drive that is possibly going bad… for example, can i check if you click on any drive bays within dashboard, (or in the drive info dropdown menus depending on dashboard version), can you see any messages for the drive, such as “warning” or “healed”?