I've been a WLS customer for years now and have moved several times from plan to plan as a result of their incredibly tempting periodic offers.
Even though they don't advertise refunds, Joshua in their CS team has been great on a number of occasions, refunding the full quarterly bill when he technically shouldn't have done.
Unfortunately, I have hit several issues with high IO - from 20 to 50-plus percent iowait in top - resulting in an SSH login taking upwards of 30 seconds, and rendering the entire VPS pretty much unusable.
After raising numerous support tickets - each time resulting in someone supposedly making a very non-specific change and claiming the issue is fixed, I can only surmise that their London DCs resources are hugely overcommitted.
I'm unsure if these issues persist into other datacentres, but unfortunately their London location is pretty consistently bottlenecked and I'd much rather give someone else a go than fight things out.
Update - just been doing a bit more digging, it seems likely that the node is swapping quite a bit, as I have 361MB in use (6GB available) and 227MB in swap! One of my processes is also swapping 99% of the time according to iotop, so the culprit looks like overcommitted RAM.