If your O2 sensor is playing up, then it may be going into limp home mode/ reduced power or even very lean settings of some capacity. This would explain why you are experiencing low performance. Also remember though that the variator is mechanical form albeit more primitive versionof VTEC but based on oil pressure and a solenoid switch to determine its operation. When it stops working as it should, you can also experience up to 20-40 bhp drop in power output and torque. Combine both together and it's got only 120bhp from out the factory gates brand new, there's your explanation mate.
Other down sides are considerably more oil burning, which could probably have coated and clogged the o2 sensor probe!
Remember, there is always, always an initial cause to mechanical and electrical problem on cars, they don't want to run badly. Ignore the original cause, suffer the aftermath further along the car's systems and more often than not, they are bigger more expensive items along the way and time line....
For example : split CV boot left alone, leads to hub/bearing and CV joint issues, leave them alone, then we wonder why a diff explodes or a wheel passes us on the motorway and we wonder where the pretty sparks come from.
In short... The longer we leave faults, the more they multiply and expensive and drastic they become!
Hope this helps mate.