Hi,
Loading 400 items at the same time via the plugin? I am not sure if that is a good idea whatsoever. That is not the same as searching for a few dozen of items from a potential list of hundreds of thousands of posts. Pushing that many elements to one page will definitely spike the browser perfromance, it is no longer an issue that the plugin has to handle. The actual search may only take a fraction of second, but then parsing all the item titles, requesting the images from the database etc.. can be really painful to any server, and then displaying all that information in the browser is a task for the client PC.
There is actually a built-in lazy loading script that you can enable, it will not load the images until paginated or in viewport. I would also strongly recommend enabling caching, it will help tremendously for displaying 400 items at once.
There is also an automatic image cropping script that creates the closest image thumbnail sizes to the results.
You can also define the preferred image source sizes under the General Options -> Image options panel.