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I would like to test this with values greater than 100, like 1000 or 10,000. Can you explain in some detail what this function does, and what the value means?

This value simply sets the delay bewtween updates. 100 is the maximum value beyond which there is little improvement, so it will not be increased beyond that.

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There is one scenario where no meaningful speed improvement is detected. That's when echoing a flood of text that contains various unicode symbols

Yes, this indicates that the font linking is the bottleneck in this case. This cannot be optimized further at this stage. In future, mIRC may switch to a more modern and sophisticated method of displaying Unicode. This used to be Uniscribe but that has been replaced by DirectWrite. Unfortunately, both have their own issues. This would require a complete rewrite of the display routines. However, it is possible that using a newer method could make the display routine slower. There is no way to know without first implementing it, which would take a lot of work.