Suggestion required for performance

We are having very slow website load times. Using JMeter, with 5 users, site load times goes to 10 seconds per page.

We have used codes like this at multiple places:

using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["CMSConnectionString"].ConnectionString))
    {
        SqlDataAdapter adapter = new SqlDataAdapter(sql, con);

        DataTable dt = new DataTable();
        adapter.Fill(dt);

        repeaterCaseStudies.DataSource = dt;
        repeaterCaseStudies.DataBind();
    }

Question: Is above code optimized? Should we use SqlConnection object as static i.e. singleton?

Sometimes the queries are complex, and hence, need to write custom codes in web parts.

Any other suggestion?