theme speed is slow - Theme Name ONUM

Hello, ThemeForest pros.
I have a theme slow speed Issue. I was looking for my industry-related theme.
But when I’ve bought that theme and started work on it this theme slowed down my website (
I bought WP Fast Cache Premium plugins and Smush Pro for Images Optimization but I’ve noticed that the premium plugin didn’t work as I was expected It’s worked little improvement.
Thank you all for your time.