Just the way it's coded most likely, using lua and loading a bunch of files from hd during the render event.
Ashita's uses the images already loaded into memory for resources and it doesn't have any of those issues.
To further elaborate, since it's drawing during render event the next frame can't start until it finishes and if it needs to access the hard drive to pull a new picture it will slow down the render event and cause a framerate drop as that frame waits for the picture to be ready.