Below is sample code:
StopWatch iFrameload= new StopWatch();
iFrameload.start();
List<WebElement> ele= webdriver.findElements(By.tagName("iframe"));
System.out.println("Total frames: "+ ele.size());
iFrameload.stop();
long iFrame_loadTime_ms= iFrameload.getTime();
long iFrame_loadTime_Seconds=iFrame_loadTime_ms/1000;
System.out.println("iFrame load time: " +iFrame_loadTime_ms+" "+ "milliseconds");
System.out.println("iFrame load time: " +iFrame_loadTime_Seconds+" "+ "seconds");