Increasing analogread speed on the Arduino Uno I want to transfer data from my smartphon to the Arduino by using the audio jack of my smartphone. For this I need verry high reading frequencys.

I have used

bitClear(ADCSRA,ADPS0); 
bitSet(ADCSRA,ADPS1); 
bitClear(ADCSRA,ADPS2);


to boost analogread to ~10microseconds, which would be enough. But somehow I am still wasting 5.4 ms at the beginning of the transmission.

The only thing between signal start and first analogread is a start of a for loop:

while(analogRead(A0)&lt;50) (true); //wait for start of transmission
for(i = 0; i&lt;500;i++){
data[i]=analogRead(A0);
}


But does starting a for loop take 5.4 ms? 
Do you have any ideas?
It sounds like you could benefit from interrupt handled free running sampling, doing that should get you sampling rates up to the maximum listed on the 328's datasheet, 76.9 kSPs. Although, that's at a reduced resolution. This site explains better, under the section "Free running sampling".