Unexpected SCK behavior

[Ouroboros75]

I'm trying to establish a Pi 3b+ as an SPI file transfer master
Here's the code i'm using:

`#!/usr/bin/python3
import spidev
import time
import binascii
import RPi.GPIO as GPIO
import crc16

spi = spidev.SpiDev()
spi.open(1, 0)
spi.max_speed_hz = 42000000
GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.OUT)
GPIO.setup(18, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)

buffSend=[]
for i in range(4192000):
buffSend.append(0x48)
print("start")
print(len(buffSend))
try:
spi.writebytes2(buffSend)
print("\nsend")
except KeyboardInterrupt:
spi.close()
`

The resulting SCK wave form is:

This is rather confusing to me. From what i understand, after each 4 bytes transferred there's an unusual gap until the next 4 bytes.
Is that correct? If yes how can i remove this gap.
Any help is GREATLY appreciated.

• Kernel version: 4.19.118-v7+
• cat /etc/os-release:
  PRETTY_NAME="Raspbian GNU/Linux 9 (stretch)"
  NAME="Raspbian GNU/Linux"
  VERSION_ID="9"
  VERSION="9 (stretch)"
  ID=raspbian
  ID_LIKE=debian
  HOME_URL="http://www.raspbian.org/"
  SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
  BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"