the speed might fluctuate a bit, especially when you start using them as the drobos do a bit of self-optimising.
once things setting down again, if you try some more tests it might speed up.
the drobos also do some extra data verification, known as data scrubbing, which helps protect the data i think once a month. while this probably isnt going to affect things much now, it may happen in future (but its actually for the best, and as far as i rememember, it will run more in the background if the drobo is being used at the time though)
for the vibrations, i have had very good success with my drobos, especially the first 2 i got, after putting each foot of the 4 tiny feet that the drobo units have, on top of a piece of cork or something similar like a soft, thick drinks coaster, of the same height for each foot. (even if the drobo plastic feet have some rubber on them, the extra cork really reduced the vibration noise for me).
also, i happened to put an object on top of my gen 1 (something that was heavy enough like a hard drive equivalent in weight) and that stopped any extra vibration).
maybe some cork / coasters can help you too?
also, if your tabletop is made of a thin piece of wood, that can also resonate, and funnily-enough, that is actually something i still need to address with my 5D now that i moved it a bit, but once i find something of about half a kilo in weight, that should do the trick there too
As for the speeds i don’t think thal 60/90 mb/s can be attributed to some settling down proces.
I have the same experience with the vibration putting something soft under the feet helped, and pushing on it from the top also helps but sometimes that front magnetic cover starts to vibrate like crazy. If i decide to keep it i will do a full sound dampening like i did with a pc but for now it’s hard to keep it on for long time. Using it on the desc was unbearable it immediately went on the solid flor
ah ok no problem,
for me i have never had a rattling cover, so maybe a thick, secure wedge of folded card just to lock the side or bottom of the cover between the base plate could be all that is needed to stop that, or to check in case one of the magnets is not quite secure in its slot, (but please be careful in case the card can go inside and touch a hard drive or something as they can get really hot)
the floor is probably ok (though if you happen to have a fluffy carpet, it might be good to elevate it a bit, such as by putting your drobo onto volume 1 of the british encyclopedia - that ought to to the trick. (or any volume 1 of a tax or law book too)