.NET 10 Performance: The O(n^2) String Trap and the Zero-Allocation Quest

.NET 10 string concatenation using += creates O(n^2) performance degradation, generating massive heap allocations (379.4 MB at N=10,000) and GC pauses. While StringBuilder is optimal for small-scale operations (N=10) due to runtime tuning, a stack-based ValueStringBuilder using ref struct and stackalloc achieves zero-allocation predictable performance at scale. The article demonstrates that choosing the right string manipulation strategy depends on operation scale, with over-engineering small cases being as harmful as ignoring scalability in large ones.