Is it correct to say, "Almost all workload is done by the server computer"? What I wonder about is the almost at the beginning of the sentence, and the usage of done. Is this suitable for a formal letter?
"Workload" is a mass noun. Therefore, you would say, "Almost all of the workload is done on the server." If you wanted to use a count noun, you could say, "Almost all work is done by the server."
However, the phrase fragment "Almost all" is colloquial. For formal speech and writing, try "A majority of the work is done by the server." Further, "is done" is also colloquial. Try, "A majority of the work is performed by the server." In rough numbers "a majority" is anything from 50.1% to 99.9% There is no implied size beyond being the major portion of something. "Almost all" is perhaps less vague, but in professional writing, it sounds like you're avoiding the numbers.
In the first paragraph, I changed "on" to "by." When speaking of computer processing, there are regional uses of "on." Using "by" is less regional and more widely accepted. "I worked on my car" would imply that I repaired my car. If I said, "work was done on my car," I would also be implying "repair." If I said, "my car did all the work," I'm implying that the car's operation is a "work" of some kind. I could rephrase this to "all the work was done by my car." This latter example is similar to the usage in computer operations.
You would either say "Almost all work is done (or carried out) by me (a person, not the server)." or "Almost all workload is carried by the server (could also be used for a person)." If the work is already finished, of course use past perfect (has been done).
Edit: I wasn't sure about the usage of "handle" and "carry" a workload after thinking about it further. So I checked Ngram. I'd say it's a par.