OS X Yosemite Wi-Fi Issues: Slow and Unreliable

Since the launch of OS X Yosemite on the 16th of October the internet has been awash with complaints about Wi-Fi performance. The issue seems to be widespread (although we personally haven’t had any problems yet) with reports coming from MacRumors and Apple’s own Support Communities.
Complaints detail extremely slow Wi-Fi performance leading to long load times as well as unreliable connections leading to constant disconnects. Issues seem to affect various systems and locations with no-one appearing immune. Numerous fixes have been suggested but there is no ‘one size fits all’ solution. This has lead many to conclude that it may be a Yosemite issue that Apple will have to deal with in a future update.
Possible fixes include disabling Bluetooth, switching off Handoff, using a new network location, re-installing Yosemite, using “G-mode only” on your router, disabling Dropbox syncing, turning off WPA router encryption, and disabling all proxies in Network Preferences. For many users, however, none of these fixes seem to solve the problem. It does seem that swapping to a 2.4Ghz band does work for a large number of users, however.
Official Apple solutions include removing excess preferred networks and resetting the System Management Controller.
OS X Daily has provided a great guide on fixing Yosemite Wi-Fi issues, check it out here. If problems can’t be fixed they recommend downgrading to Mavericks until Apple have fixed the issue as a last resort.
If you’re going to attempt a technical fix we recommend backing up ahead of trying out your chosen solution.