hello,
In terms of SEO, I don't think having this as a parent portal will cause any issues whatsoever.
The one thing I would ensure is that each portal uses its own unique domain name related to the topic of the domain name, ie. www.gardentools.com, www.roofingtools.com (ie. not a child portal)
Keywords within a domain name do have an affect on search engine rankings, so it is best to have your main keyword in your domain name.
There should also be no issues with speed.
The one thing that you may want to consider is personally if I am creating a website on a different topic, I set it up in its own unique DotNetNuke installation, this means that if one of the websites goes down / has an error / becomes corrupt, the other domains will not be affected, whereas if you have several hundred domain names within one DotNetNuke installation and you have a problem with the DotNetNuke installation all of your domain names go down and this could potentially mean that all of your "money making" websites are not able to make any sales.
Splitting your website up like this minimises the risk on your potential income.
You may find it useful to view this tutorial:
DotNetNuke and Multiple PortalsThanks,