Guy i have seen strange ass behavioir as well. Ive never been able ro abover 6-7mhz with 1500+ leds. 1000 i can drive at 10mhz but anything above that na
@Stefan_Petrick today’s real life observation: I have a large installation with 4 strips, total of 800 APA102 LEDs. Cable lenght varies from 0.4m to more than 2m. On the lab bench I used 2m of 4x.35mm² straight, shielded wires - and got nasty flickering.
Was able to eliminate it with a 51 Ohm resistor. Because I needed .75mm² power supply cables I used twisted pair Data and Clock. Installed it with a Adafruit Feather Huzzah (3.3V) w/o resistors or caps - and it works right out of the box.
The twisted pair are DYI: Two cable-ends in a vice, the other end in a cordless drill, and then slowly turn it - and most importantly - massage it a bit, and you get a perfect twisted pair.