HI, I'm working on a project of 25 meters DotStar strip (30 LEDs per

I’m working on a project of 25 meters DotStar strip (30 LEDs per meter)
When I run the demo code (strandtest) on one meter numLEDs = 30,
I experienced a normal speed between each other LED lighting.
But when I run the same code for 25 meters (numLEDs = 750), there is a slowness between Led to each other.
Is mandatory parameter numLEDs?
If so, why?
What can be done to not experience the slowness for a long LED strip ?

numLEDs is just a constant that you define at the beginning of a sketch that holds the total number of LEDs in your setup.

That specific number is always necessary somewhere in you sketch. You could rename it to anything you want but that name is quite an accurate description.

Because of the nature of these addressable strips, it may be normal to notice some slowness with long strips. However, these dotstar strips are incredibly fast and you should be able to update the whole string of 750 DOTStars a few hundred times per second.

That demo code must have a delay in there somewhere… Just remove it or decrease it’s value!

Which code are you running? There’s no example in FastLED named strandtest. Please post the code you are running to pastebin or gist so I can see exactly what you have going.

You haven’t included information about what hardware you are using (uno? Teensy?) Note that if you are running the LEDs on a pair of pins that do not have hardware spi, it will be bit banging at a rate of a about 1Mhz, give or take, on an arduino (might be a bit faster, haven’t benchmarked in a while). That means that 30 LEDs would take about 1ms to write out, while 750 LEDs would take about 25ms to write out.

I have a project that uses 788 WS2812 LEDs. It slows down considerably because it just takes time to address all those LEDs. This is where a Teensy can help out a bit in that you have a faster processor for all your animation code. I will be working to put that into a multistrip approach soon. I have 5 logical sections so will need to define over 1500 LEDs but the parallel output should make it 5 times faster.

“strandtest” is a demo from Adafruit and it’s problems don’t belong to FastLED.