Domain Vs. Sub-Domain

Within user Tools > Domain Mapping, setting Primary to http://example.com renders blank page when visiting http://example.com. However, setting Domain Mapping Primary to http://example.mainsite.com and visiting http://example.com the site comes up, but the URL in the browser is http://example.mainsite.com.

How do I get the URL in browser to only show http://example.com? And why do I get a blank page when setting Primary to http://example.com?

Web site front end and back end have slowed considerably with the install of the DM plugin – is this expected?

https://ww.wp.xz.cn/plugins/wordpress-mu-domain-mapping/