Drupal 8 get image dimensions by applying image style effects. This doesn't cause performance issue because the dimensions are calculated without creating the actual image.

file.php

$image = \Drupal::service('image.factory')->get($file->getFileUri());
$image_style = \Drupal\image\Entity\ImageStyle::load('gallery_large');
// Set source image dimensions.
$dimensions = [
  'width' => $image->getWidth(),
  'height' => $image->getHeight(),
];
// After calling this, the $dimensions array will contain new dimensions.
$image_style->transformDimensions($dimensions, $file->getFileUri());