Internal Hardware
Powering the Synology DiskStation DS1515 is the Annapurna Labs Alpine AL-314 SoC clocked at 1.4GHz. The ARM Cortex-A15 quad-core processor supports DDR3, DDR3L and LPDDR2 memory and Synology has gone with a single 2GB DDR3-1600 memory module, which isn't easily upgraded, at least not without voiding the warranty.
Keeping the Alpine AL-314 SoC cool is a small aluminium heatsink which lacks a dedicated fan.
It has become relatively common for today's NAS devices to use SO-DIMM slots, especially for higher-end models as this allows for upgrades to be made. Unfortunately, while the DS1515 does use SO-DIMM memory modules, there is just one DIMM slot which is of course occupied with a 2GB module. Furthermore, replacing it with a larger module will void your warranty.
Providing the quad Gigabit network are four Realtek RTL8211DN controllers that support IEEE 802.3az -2010 (Energy Efficient Ethernet), Wake-On-LAN (WOL) and of course Synology has enabled Link Aggregation as well as Failover support.
The only other noteworthy controller is a Marvell 88SE9170 SATA controller which provides two SATA 6Gb/s ports, Synology use this to provide the dual eSATA expansion ports. This means the expansion unit should have at least 500MB/s of bandwidth (750MB/s peak), which is going to be enough for an additional five hard drives.
Presumably the Annapurna Labs Alpine AL-314 doesn't offer native USB 3.0 support as Synology has had to implement the EtronTech EJ168A USB 3.0 host controller, which provides two ports with up to 2.5Gb/s of bandwidth per port.