Not all niches turn out to make millionaires, not all will succeed, but I can tell you making the best choice at the start can save you months of frustration!
The best ways to manage your site speed are right in front of you, to speed up your WordPress site you need to manage hosting, plugins, theme, and images.
If you want a solid evergreen niche then Bonsai Trees are ripe for a nice long term site build where you can gain ground but have it rank for years to come.
Many people will tell you that your posts should always be skyscraper posts that are 5000 words or more, this is incredibly false and you need targeting instead
What you need to do the most to be successful is to learn how to find the best keywords and phrases through good search analysis to get your content ranking.