Internal Clock and output frequency

[chopnhack]

Hello all, I am investigating the use of a PIC12F675 to produce various frequencies on one of its GPIO's. I am aware and have experimentally confirmed that the INTRCCLK will output a ~1MHz signal on pin 3. I wanted to see if I could get a range of frequencies by simply toggling a port on and off. I got some unusual results that I wanted to see if anyone knows why they would occur. It seems as if there is a ceiling of ~187KHz. Results are in the table below:

XTAL 4000000	1ms	1.1KHz
	0.1ms	9.69KHz
	0.01ms	64.2KHz
	0.001ms	158KHz
XTAL 8000000	0.001ms	136.8KHz
XTAL 1000000	0.01ms	187KHz
XTAL 500000	0.1ms	58KHz
	0.01ms	158KHz
	0.001ms	187KHz

The code used is below, the define _XTAL_FREQ line was changed for each set of runs:

// CONFIG
#pragma config FOSC = INTRCCLK  // Oscillator Selection bits (INTOSC oscillator: CLKOUT function on GP4/OSC2/CLKOUT pin, I/O function on GP5/OSC1/CLKIN)
#pragma config WDTE = OFF       // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF      // Power-Up Timer Enable bit (PWRT disabled)
#pragma config MCLRE = ON       // GP3/MCLR pin function select (GP3/MCLR pin function is MCLR)
#pragma config BOREN = OFF      // Brown-out Detect Enable bit (BOD disabled)
#pragma config CP = OFF         // Code Protection bit (Program Memory code protection is disabled)
#pragma config CPD = OFF        // Data Code Protection bit (Data memory code protection is disabled)

#include <xc.h>

#define _XTAL_FREQ 500000

 void main (void)
 {
  // SET UP
  TRISIO = 0b000010;          //Only AN1 is a input all else are outputs.
    
  while(1){
      GPIO = 0b000000;        //Off 
      //__delay_ms(1);
      GPIO = 0b100000;        //GP5 On 
      __delay_ms(0.001);
  }

The scaling from 1ms to 0.1ms is nearly perfect with 4MHz intrcckl - 1 to almost 10KHz, but it seems to fall off from there. Any thoughts? Thank you all in advance.