I like @zorce think that talking about new, trendy technology, and with static cache you can achieve great results with WordPress.
Just recently I decided to connect Gatsby to WordPress, to distribute a few posts, then realized that WordPress extra link, as the task was too trivial, and the choice fell on Markdown, front, Gatsby, deploy + lighthouse check on Gatsby cloud with on Netlify...
example
conclusion for simple solutions for Gatsby - WordPress is not needed
for the speed of a WordPress site - do not need Gatsby
Now about the cost:
To work with Gatsby need someone who is well versed in the React + HTML/SASS/SCSS, that is, even a few people, and then it depends on the number of necessary components... Are developed are not cheap... Roughly 5 templates can take anywhere from 50 hours to, say, the price tag of such a specialist$ 30 per hour...
In order to optimize WordPress for a few hours, and you can get a decent result, everything else, such as as a Frontend of your Theme, depends on the desire to optimize all under type-check Google Page Speed Insights = Lighthouse, and can take tens of hours...