Unfortunately none of the solutions above solve the issue. The good news is that it only exist with Sorrowww's Sin City mod and I've talked to Sorrowww in Discord and he said he will be fixing this lag issue by breaking up the dialogue to load only what’s needed. So hopefully we will see some improvements in his future updates.
Currently I switched to "Slave Bazaar" and it is running smoothly. This only proves that "Sin City" is heavily package and it might be too much for a flash player to handle.