Hello Rjo80, Yes if you have already replaced the failed drive it is ok to still use the array as you are correct that you may get a little slower on performance until the rebuild finishes.  Once the rebuild is complete then the array will go back to normal speed. Let me know if you have any other questions.
I had a drive fail in my PowerVault 220s, and it's not rebuilding it onto the hot spare.  is it ok to still use the array while it's rebuilding?  If i recall correctly, its ok to still use it, but performance will be a little slower due to the rebuild going on in the background.  is that correct?
Rebuilding a failed drive, is it ok to still use while rebuilding?