When power is lost on an AVR chip (4313 and 8535) why does the mcu run slower?
That thing you call a reset button isn't.

Switch bounce will likely be issuing multiple resets to your MCU milliseconds apart and possibly of microsecond duration, and possibly spending time in indeterminate logic states.  You should check the datasheet for needed reset timings.  My hunch is this spurious multiple out-of-spec reset pulses is putting the MCU into an undefined state - and apparently not operating per its fuse configuration, and instead running off its internal 8MHz oscillator*.

Standard practise is to at least have a cap to ground, at least 100n, but more likely 1uF..10uF depending on the reset timing specs and, more importantly, the bounce characteristics of the switch, which normally have to be estimated empirically (i.e. watch what RESET is doing with on an oscilloscope).  Putting the switch/pull-up/cap through a schmigger can help a lot too.  But normally a proper reset controller chip is used to deal with the reality of switch bounce in imperfect mechanical switches.


This is in the same realm of the fuzzing of power rails and other critical signals - like RESET - used by hardware hackers to put the MCU into certain states, for example to read out code, when that read-out functionality should not be possible due to fuse config.