add note of extraction time to README.md (#6630)

Author: kinh-nguyen

README.md

@@ -60,7 +60,7 @@ Pre-process the extract with the car profile and start a routing engine HTTP ser
 
     docker run -t -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-extract -p /opt/car.lua /data/berlin-latest.osm.pbf || echo "osrm-extract failed"
 
-The flag `-v "${PWD}:/data"` creates the directory `/data` inside the docker container and makes the current working directory `"${PWD}"` available there. The file `/data/berlin-latest.osm.pbf` inside the container is referring to `"${PWD}/berlin-latest.osm.pbf"` on the host.
+The flag `-v "${PWD}:/data"` creates the directory `/data` inside the docker container and makes the current working directory `"${PWD}"` available there. The file `/data/berlin-latest.osm.pbf` inside the container is referring to `"${PWD}/berlin-latest.osm.pbf"` on the host. Noting that this process can take a long time to complete with little changes on the terminal output, for example, a Mexico's OSM file of 550.7MB took around 30 minutes to finish extraction and generate edge-expanded graph representation.
 
     docker run -t -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-partition /data/berlin-latest.osrm || echo "osrm-partition failed"
     docker run -t -v "${PWD}:/data" ghcr.io/project-osrm/osrm-backend osrm-customize /data/berlin-latest.osrm || echo "osrm-customize failed"