Accord '07 VTI-L Sluggish Performance

I've got an Accord 07 VTI-L, which has just had its 22k service. It has now done approximately 25k and for the past couple of weeks, the car has the following symptoms:

• Engine acceleration noise - Sounds like it is straining the engine when accelerating; and
• Struggling to accelerate (Eg. 50k+), eg. slower performance

I checked the engine oil, seems to be 'fresh' in terms of the colour (pinkish). Not too sure what to do next. I'd imagine the air filter would have just been changed, but I will take a look and see if its dirty. Where is the best place to purchase the air filter replacement (and the cabin filter) from? Supercheap?

I checked the exhaust (near the tip) and there isn't any holes.

Any pointers/advice you can give me, I'd appreciate it.

Thanks.

What sort of fuel did you put in it?

Just the standard - Unleaded. That's the recommended type of fuel to put in for the VTI's.