The phrases ‘worse than usual’ or ‘better than usual’ are very common when comparing typical performances – but notice how in English we use the adjective, usual, not the adverb, usually, like in Czech.
The same is true of normal too, e.g. My performance was worse than normal, his behaviour is worse than normal. The words usual and normal in this context mean the same thing.
It may seem like a small mistake but it is a very common one and when such a sentence is said incorrectly, it really stands out as a bad mistake for native speakers.
The weather in July is worse than usual. That said, the amount of rainfall is better than normal.