Yes—this is normal, and it’s not specific to your camera.

A camera can shoot at full burst speed only until its internal buffer fills. After each shot, the camera must process the data and write it to the memory card. If that pipeline slows down, the buffer fills sooner and the burst rate drops.

Higher ISO can reduce burst depth for two main reasons:

1. More processing work: At high ISO, images contain more noise, and many cameras apply more noise reduction or related processing. Even RAW capture may still involve processing and compression before writing.
2. Potentially larger/harder-to-compress files: Noisier images tend to compress less efficiently, so RAW files can be slightly larger and take longer to write.

So the limiting factors are the camera’s image processing speed, buffer size, and card write speed. At low ISO, the camera may clear buffer data fast enough to sustain more frames; at high ISO, it may not.

This happens on many digital cameras, including higher-end models—the difference is usually just how much the burst depth or frame rate is affected.