Hi @npsantini
The arduino UNO, nano, mini, (ATMEGA328) when connected to 5V, has a clock of 16 Mhz (16,000,000 Hz).
It has a cycle of 62.5 peak seconds or 0.0625 microseconds.
As, in simple terms, it executes one instruction per clock cycle, so theoretically it would have a pulse every 2 cycles.
1 cycle turns the LED on and one cycle turns the LED off.
In other words, we would have the LED blinking every 0.125 microseconds.
But as when writing the program other instructions are added, I would say you can get 1 pulse on the LED every microsecond. Or 1MHz.
RV mineirim
PS:
Pretty much any LED available can be operated at far higher blink frequencies than 1 KHz: White LEDs or others which use a secondary phosphor would be the slowest, often topping off in the 1 to 5 MHz region, while standard off-the-shelf primary LEDs (red, blue, green, IR, UV etc) are typically rated at a cut-off frequency of 10 to 50 MHz (sine wave).