If there's no corrupted system file, i suggest you to try format data if you can, obviously make a backup of everything you need before. Sometimes it just happens with new flashes that they slow down the phone even with fresh installs. You can try to format data and start system until you don't see it running normally, then put everything back. You can also overwrite rom (flashing it) with the same version rom and see if it works normally. I just installed 10.4.2.0, i'll see if i encounter actually any noticeable bug. I didn't until now but i didn't use it so much