Great question — answering as the UphillTech team:
I assume they utilize prefix caching, which means that mathematical operations needs to be carried out only once for each unique system prompt (or prefix). The per-prompt processing happens only after the common prefix. But since you think that processing the system prompt only takes some microseconds, did you already know about prefix caching? Integrating the system prompt directly into weights is an interesting idea, I'll let others answer that one.
Full answer by NikoNyrh: https://ai.stackexchange.com/a/50265 (CC BY-SA 4.0).
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