CB400SF Hyper Vtec Spec 3 Sluggish performance @ 5000 rpm onwards

my 2005 CB400SF is experiencing sluggish performance at 5000rpm onwards. I deemed it sluggish because the sound is not as "smooth" as before. It behaves like how a rev limiter would. Vtec can still be engaged.

I have tried the following inspection and checks:

1. Check for blown fuses
2. Replace all 4 spark plugs
3. Vent out residual pressure from fuel tank.

I guess my next step would be to clean the carburetors if it helps.

Some bike information for reference

My chassis number: NC39
My engine number: NC23

Thanks for the help!