Hi,
Do you already use the lates 9.7.0 version? In this version the size of emails built with our editor has been reduced by 25% on average, which should improve the sending speed.
Other then that I would reccommend to contact your hoster and check if they offer some kind of database caching. Database caching involves caching the results of database queries on the server, so that subsequent requests for the same data can be served quickly without having to re-run the query. This can improve the performanceby reducing the time required to fetch data from a database.