Why the window is a range
The standard window is 2–4 business days. It is quoted as a range because two documents of the same page count can be very different jobs. A certified translation for Latvia of a one-page marriage entry is close to the short end; a bundle of school records with tables and footnotes sits near the long one.
What actually eats the time
Three things do most of the work. First, reading: nothing is written until the whole sheet has been read, because a heading at the foot can change how a box near the top is rendered. Second, layout: a table has to be rebuilt so the two sheets can be read across. Third, the second pass, where the Latvian is compared with the image line by line.
A certified translation into Latvian in London does not gain much from more hands, either. Splitting one document between two people creates a matching problem at the end, so a single translator normally carries a job from the first read to the signature. That is a deliberate limit on speed, and it is what keeps the wording consistent from the first page to the last.
The part of the calendar you hold
The clock starts when the last sheet arrives, not when the first one does. Sending a set piecemeal is the single most common way a short job becomes a long one, because the reading pass has to be repeated over the whole bundle. An official Latvian translation in London booked as one complete set almost always lands at the near end of the window.
The rate, and the paper you keep
The cost is about £25 a page and does not change with the date you choose, so a fast job and a slow one of the same length are billed alike. Where a birth, marriage or death entry is involved, Westminster Register Office at 317 Harrow Road, London W9 3RJ issues certified copies, and a certified copy can be imaged and translated while your own paper stays at home. A certified translation in London for Latvia never needs your original in the post.