The processor and RAM are the same, so "performance" in that sense should be similar; as you may be aware, the biggest differences are Z-Wave (much bigger difference between the C-5 and C-7, but also a slight upgrade between the C-7 and C-8) and Zigbee (from the C-7 to C-8; C-7 and earlier are mostly the same, aside from external radios you may find on older models). If you notice improvements, it may be from any of a few things:
- 700-series Z-Wave and 800-series Z-Wave has faster processor speed and increased range compared to older revisions (though the Z-Wave network maximum speed is the same as 500-series)
- C-7 and later support Z-Wave S2, SmartStart, and better network troubleshooting tools (per-node repair and replace, network topology, and other statistics on the Z-Wave Details page that are not available on C-5 and earlier), if any of that is important to you -- see the Z-Wave Details docs for what this looks like on the C-7/C-8
- external antennas on C-8 are likely to give more direct connections to the hub with fewer repeaters/routers, which may improve "performance" in that sense
- a few more Zigbee options on the C-8 (e.g., power level), along with a new Zigbee 3.0 chip, but I wouldn't say that the differences with Zigbee are as drastic as they are with Z-Wave
Other changes you might be aware of:
- internal Wi-Fi radio as an alternative to a wired Ethernet connection, for either regular use or initial setup, though wired is still recommended (possible after setup with a dongle on C-5/C-7 as well)
- ability to migrate Z-Wave and Zigbee to a C-8 (from any model hub with internal radios) if you have Hub Protect or as part of a free one-time C-5/C-7 migration; only Z-Wave can be migrated to earlier models
Those will probably not have much of an effect on your daily use.
Personally, I have a lot of Z-Wave devices and find the Z-Wave changes since C-5 worth it, but I can't say whether you would. 
There are certainly a few new features, though, especially with Z-Wave, beyond just the hardware changes.
