See https://core.trac.wordpress.org/ticket/61782#ticket

test-wp_get_wp_version_memo.php

<?php
/**
 * Plugin name: test wp_get_wp_version performance.
 */

if ( ! function_exists( 'wp_get_wp_version' ) ) {
	function wp_get_wp_version() {
		require ABSPATH . WPINC . '/version.php';

		return $wp_version;
	}
}

function wp_get_wp_version_memoed() {

	static $wp_version;

	if ( empty( $wp_version ) )
		require ABSPATH . WPINC . '/version.php';

	return $wp_version;
}

add_action(
	'wp_head',
	function () {
		if ( empty( $_GET['test'] ) ) return;

		$its = 1e4;

		$t = hrtime( true );

		for ( $i = $its; $i--; )
			wp_get_wp_version();

		echo number_format( ( hrtime( true ) - $t ) / 1e9, 5 ) . 's', "\n"; // 0.11526s

		$t = hrtime( true );

		for ( $i = $its; $i--; )
			wp_get_wp_version_memoed();

		echo number_format( ( hrtime( true ) - $t ) / 1e9, 5 ) . 's', "\n"; // 0.00040s

		$t = hrtime( true );

		for ( $i = $its; $i--; )
			wp_get_wp_version();

		echo number_format( ( hrtime( true ) - $t ) / 1e9, 5 ) . 's', "\n"; // 0.11483s

		$t = hrtime( true );

		for ( $i = $its; $i--; )
			wp_get_wp_version_memoed();

		echo number_format( ( hrtime( true ) - $t ) / 1e9, 5 ) . 's', "\n"; // 0.00060s

		exit;
	},
	0,
);