@ jerry
Are you trying to load all 603 on the same page? I’m not sure that’ll work. I recommend no more than 50, max which is why all the templates which support pagination default to 20 per page. This is for the same reasons WordPress defaults to 5 (or is it 10) posts per page. You’ll run into page loading issues.
Things to try …
Increase the memory assigned to PHP, normally I suggest 128MB, but you may need more, so try 256MB.
You’ll also need to increase PHP max execution time. Generally 30 seconds is enough but I’ve found to 60–90 seconds is often needed when plugins are added to WP. With the higher number of entries, you may need an even higher time limit.
After you have it working, Then I would highly suggest a caching plugin so the page only needs to be created once in awhile. That can greatly increase the page load time. Currently I’m using WP Rocket which I’m quite happy with.
Hope that helps! let me know.
