When you choose manufacturer in default prestashop site — you get list of all products of this manufacture, without any grouping. Module blockmancategories add a list of categories into manufacturer page.
In order to improve site performance I implement Cache module for my shop, which based on prestashop.
Cache module based on Pear:Cache_Lite engine and able to store cached part of pages to file system.
My estimation in such performance improvement — 2-8 times, depends of you site and database.