C# “StringBuilder Turns Slow” — The Hidden Capacity Trap

Everyone uses StringBuilder for speed…
but almost nobody knows WHY it becomes slow after heavy use.

When StringBuilder grows beyond its internal buffer → it doubles memory and copies everything.
Large loops = huge perf waste.

var sb = new StringBuilder(50000); // pre-allocate

If you know approximate size → pre-allocating saves massive CPU.

Don’t ever do:

sb.ToString(); 
sb.Append(...); // ❌ forces buffer copy twice

Call ToString() only at the end.

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