Translation adds a cost the first time a page is seen in a language, and effectively none after that. Understanding which of those you are measuring explains most reports of a slow site.
Where the cost actually is
First view of a page in a given language. The text has to be translated. This is the slow one.
Every view after that. The translation is stored, so the page is assembled from stored text with no translation step. This is fast, and it is what your visitors overwhelmingly hit.
A page that feels slow on your first check and fine on the second is not a problem. You measured the first translation.
One thing to be clear about: later views skip the translation, not the request. Every uncached view of a translated page still makes a round trip to Universally, so there is a small, constant latency cost per page view. A page cache is what removes that, because a cached page is served without contacting Universally at all.
Make the first view rare
Since the cost is per page per language, the goal is for real visitors to arrive after the translation exists rather than triggering it.
Visit your key pages in each language after setup. Your homepage, top landing pages, and main navigation targets. That moves the first-translation cost onto you instead of a visitor.
Let search engines crawl. A crawler is treated like any other visitor. On a page that is already translated it costs nothing. On a page that is not, the crawler triggers the translation and those words count against your total exactly as a visitor's would, which is worth remembering if you point a crawler at a large site in several languages at once.
Caching
Page caching is the single biggest factor, and it works normally with Universally.
Enable your site cache and a CDN. Translated pages cache like any other page. Once cached, a translated page is served without touching Universally at all.
Clear the cache after changing translations. A cached page holds the translation it had when it was stored, so editing a translation or changing translation rules has no visible effect until the cache is cleared. See Clear cache and refresh translations.
Caching plugins and CDNs occasionally need configuration to treat language URLs as separate pages. See Caching and CDNs.
Reduce what has to be translated
Less content to translate means fewer first-view costs and less of your word total spent.
Exclude pages that do not need translating. Checkout, account, and admin paths. See Exclude pages.
Exclude regions of a page that should stay in the source language, such as code samples or user-generated content. See Include and exclude CSS selectors.
When it is not Universally
If translated and untranslated pages are equally slow, the cause is elsewhere: a heavy theme, unoptimised images, or too many third-party scripts. Compare the same page in your source language against a translated one. Similar timings mean translation is not your bottleneck.