Redirects are used to send search engines and users to different URLs from the ones they requested. When you are redirecting one URL to another, you should follow the best redirect Practices to retain SEO value.
If used properly, canonical links can solve the duplicate content issue. Yoast has published a long guide on the topic.
As mentioned earlier, mobile searches are more than desktop searches now. And Google is experimenting mobile first indexing and will start considering page speed as a ranking factor by July 2018. You should improve page speed of your website .
