Turn off the battery saver feature in settings. Report back if you see the problem again. The site may be causing a memory leak or something.
How much RAM do you have though?
Are you using the 64-bit version of Opera GX or the 32-bit version? What OS? Have any extensions installed? Did you make sure the RAM limited is off in Opera GX?
I tried same operation with Opera classic and i have same results, it doesn't work so i prefer use Edge browser for the moment because i didnt have problem with passwords importation.