Hello,
As for me, it is not slow, check this video – https://gyazo.com/cab1b100e0772fb39236c84b1ce0b39e Please clear cache of browser and check one more time.
Ajax requests speed depends on server response time and DB optimization structure.
“AJAX Add To Cart For Simple And Variable Products” you can disable via Theme Options, or change “Product Added Notification Type”, just go to Theme Settings and search for mentioned settings – https://prnt.sc/FbQpzDmTTrL_
Regards