Estimating the frequency at which your microcontroller is running
Take a look at the compiler output here https://godbolt.org/g/TxZSgt. Notice that the inner for loop and the asm("nop") lines correspond to 3 assembly instructions, two of which (sbiw and brne) take two cycles (for brne when taken). So instead of taking one cycle per inner loop like you predicted, it takes 5.

And actually, if you multiply your measured clock frequency by 5, it comes out to 1MHz, which is the ATTiny default clock speed as per the datasheet.


  The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source setting
  is therefore the Internal RC Oscillator running at 8 MHz with longest start-up time and an initial system clock
  prescaling of 8, resulting in 1.0 MHz system clock