Too much items in the menu

Hello dear www.remarpro.com team,

I have such a question. When I add more then 1500 item in the menu, my customizer does not work, I get 504 error.
I descreased the amount of items in the menu to 500 , and it is working ok.

But, now I have to translate this menu in the multiple languages.

And theres my question. What causes the 504 error in the customizer – to much item in single menu, or to much items in all menus combined?
Because if the answer is first, then it will be no problem for me to create multiple menus with 500 item in there,
but if the answer is second, then I dont know what to do, cause even if I will decrease my item amount to 200 items, customizer will not work, cause I want to translate into 8 languages, and 200×8=1600.

I dont know if I expressed my thoughts in a understandable way, but I swear, I tried to))

Thank you for your help,
Best regards,
Petro