Thank you so much for purchasing our theme and contacting our support center.
We suggest you to use the child theme in case you will need to add some custom PHP or CSS codes. Anyway, it will not slow down your website so use child theme in any case.
Your website speed depends on your content optimization and server performance. We can hardly advise you anything here. All we did on our server is optimized our demo content, used WP Rocket and CDN.