Yes you are correct with organic traffic this comes from organic searches and hits on your site, people entering the URL and going direct to your site is known as direct traffic, and then you have referral traffic which basically can come from any other site such as social referral, traffic from an app etc or even referrals from Google maps, shopping etc.
Of course, you can buy traffic but if you won't build up organic traffic sources over time, your business will become dependent on the amount of budget you spend on advertising. And advertising costs doesn't stay the same. Most of the time if it gets more advertisers, the costs are going up. And suddenly your profit can disappear and ROI can become negative.
There is absolutely no reason why your on-page optimization should not be perfect. It can mean the different between your website showing up on page three of the search results and your website being the top listing. We encounter so many local businesses that simply ignore their on-page optimization (or the prior SEO company didn’t optimize correctly).
A slowly loading website does two-way damage. Firstly, Google detects the crawling pace, which hurts your site’s organic ranking. Secondly, a slow speed poisons customer conversion rate and amps up the abandonment rates for your business. The second factor links back to the first one, as visitors don’t engage with your website, resulting in a loss to search engine ranking.