You have "random image" for the collage enabled. This means if an image is used that is not generated yet it takes a while.
TWG will be fast when all thumbs are created. you can generate them with the search (i started this for you once - but with 4000 images it takes ...! (normaly the browser stops after a couple of hundret images because the server is 100% in usage!))
You could do this offline and upload the thumbs ... (on thumb ~ 0.5 s - you do the math!)
OR
you turn this feature off and show the first images ony! - oy you don´t use the collage (show only one image)!
You can also reduce the number im images on the thumbs page ...
turn off autodetections then.
But you main prob is the generation of thumbs!
You should turn off the auto cleaning of the cache too - i´s in the config in the last section - otherwise images that are not viewed e.g. 40 days are deleted from the cache (only big images - thumbs are never deleted!).
I recommend turn the random image off until many people have viewed the gallery - then you can turn it back on because all thumbs do exist
/Michael