Is their a defective drive in my time capsule?

I am only getting maximum 27mbs write speeds and only 23 mbs write speeds through the drive in my time capsule. A WD green 3TB will get at least 147mbs. Most laptop rotaional drives will even get as much as 135mbs. Is thaeir a problem with the SATA controler is their a drive malfunction? Or is the new time capsule 802.11AC designed for networks running at speeds slower than a 802.11g? It just seems rather slow even for a backup, my WD 4TB external "MY book" usb 3.0 between 108-135mbs sustained read/ write, but i cannot conect it properly to the timecapsule for backup becasue it only has USB 2.0 also a technalogy well sutable for 802.11g. Let alone a firewire 400 or usb 3.0 is well sutable for 802.11n networks. My overall question is are they all built for computers in the early 1990's or is mine just defective? I don't know if i should have to return it because speed was not thought of when desiging this product/ cheap pre-mellnium parts where used in the making of this, or if i should exchange it for a properly manufacture product? Or is their going to be an update to the SATA controler to run properly. I just got the time capsule yesturday and i sware the macbook pro i purchased 3 years ago still with the original drive, runs over 5 times better than this, its hardly suitable even for a backup, let alone for a network storage solution for streaming media files to apple TV.

Please answer the question above first, if they are all this bad, than please answer the following. If i put a proper 2-3TB drive in this will it finbally get aproiate speeds or is the software/ hardware of this device limited? has anyone placed in a drive that can do 165mbs write/ read times (a good rotational drive built in the last 6 years drive will) did they get those speeds? or is the device itself have a faulty SATA conection/ controler as to not get anything faster than the 23mbs write speeds. or am i wrong completely, is there a 1980's untra wide scsi controler for the HDD, it would explain the speeds slower than 40mbs....