Learn English – “Almost all workload is done by me” — is it correct and formal

"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.