Faster way to add comma, then delete the last one

gistfile1.txt

		// Categories
		$start = microtime(true);
		$cats = get_the_category();
		if ( $cats ) {
			foreach ( $cats as $cat ) {
				$cat_list[] = '<a href="' . get_category_link( $cat->term_id ) . '">' . $cat->cat_name . '</a>';
			}
		}
		/**
		 * Filters the list of category links to be displyaed on Post's entry meta
		 *
		 * @see tujtheme_post_entry_meta()
		 *
		 * @param string $array Category links.
		 */
		$cat_list = apply_filters( 'tujtheme_filter_post_entry_meta_cat', $cat_list );
		$cat_list = implode( ', ', $cat_list );
		$cat_list = rtrim( $cat_list, ',' );
		echo microtime(true) - $start, PHP_EOL;

0.0013430118560791


		// Categories
		$start = microtime(true);
		$cats = get_the_category();
		if ( $cats ) {
			$i = 1;
			$sep = ', ';
			foreach ( $cats as $cat ) {
				if ( $i === count( $cats ) ) {
					$sep = '';
				}
				$cat_list[] = '<a href="' . get_category_link( $cat->term_id ) . '">' . $cat->cat_name . '</a>' . $sep;
				$i++;
			}
		}
		/**
		 * Filters the list of category links to be displyaed on Post's entry meta
		 *
		 * @see tujtheme_post_entry_meta()
		 *
		 * @param string $array Category links.
		 */
		$cat_list = apply_filters( 'tujtheme_filter_post_entry_meta_cat', $cat_list );
		$cat_list = implode( '', $cat_list );
		echo microtime(true) - $start, PHP_EOL;

0.00087213516235352