Please, in SefTranslate settings for your joomla site, set translate algorithm: 1 (this algorithm try divide or site to so small phrases as it can )
How we do translate:
1. Cache of phrases - every phrase we save to 1 level of cache. You can't clear it
2. Cache of pages - this is second level, it gathering from point 1. So if phrase was translated - it every time will receive from cache(not from API). This cache speed up your site, when you set cache time - you set it for this cache.
So main cost - you spend at first site translate. Next - when you add new languages, or when you add new content to site.