Yes I think so, moreover you can search for minimize WP loading time tips for,example and try the advices.
Yes, I also experience such problem, when I disable this plugin my site load time increases dramatically.
I have find some useful point. When wp admin panel is opened at the same time with my website, ajax.php causes the loading time to increase. But when I logout\close wp admin page the website loads very fast. Beacuse we, site owners always open both admin panel and website together to edit their pages 🙂
İ also experience the same problem, this ajax.php extremely slovs dovn my vebpage according to pingdom tools. What does WP thinks about that man?
