Update README.md

Author: siyuanZHANG01

docs/README.md

@@ -22,7 +22,7 @@ The raw Parquet files are not included due to size. Pre-processed JSON files are
 
 ## Usage
 
-- **Global view** — 60,000 sampled trajectories colored by speed. Use the date picker, time slider, and vessel type filter to narrow the dataset.
+- **Global view** — 60,000 sampled trajectories colored by speed. Use the date picker, time slider, and vessel type filter to narrow the dataset. To ensure smooth rendering, the vessel count shown in the global view reflects a downsampled subset of the 10% dataset.
 - **Port view** — click a port marker or sidebar entry to zoom in and see activity charts, fleet breakdown, and anchor zones.
 - **Compare** — Zoom in a port and select a second port from the "Compare with" dropdown to view side-by-side charts.
 - **▶ Time / ▶ Months** — animate through the day or the year.