Hi, I'm sorry that you have experienced "horrible performance" and problems with js methods, Drupi utilize Babel til compile ES6 and ES7 scripts into EMCA 2015 scripts, so that our compiler can understand it, the option "modern" compiling is meant to be on while developing your scripts but we highly suggest pre-compiling scripts on productions servers to speed up the loading times. Since we utilize babel nearly all native JavaScript methods should be supported, feel free to contact me through our discord
https://discord.gg/hEwaxsN so that we can find a solution to your issues.