.NET Most efficient way to replace placeholders text with actual values?

Since you will be calling .Replace() multiple times, it's probably more efficient to use StringBuilder.Replace(), since StringBuilder is optimized for multiple modifications.

If you have flexibility in the format of the placeholders, I think DotLiquid would be a good candidate for this. They probably have optimized the text processing for this scenario, although it also supports other advanced syntax so there might be overhead there.