liudr:
Then the following line must be taking a lot of milliseconds:sonar[currentSensor].ping_timer(echoCheck);
Put Serial.println(millis()); before and after it to see how much time it costs. Are you using any interrupt to catch the ping?
I don't know what code is inside the Newping.h, but the timing uses millis count rather than delays to trigger and listen for the echo. The values are stored in an array and available for access for decision making. It works very slickly for gathering distances near continuously as the robot drives forward and making turn decisions on the fly, without stopping and taking measurements.
It "looks" fast, but obviously it isn't in the context of running the web server before or after it.