FastCache is a small, high-performance, thread-safe in-memory cache library for .NET 6+. The core is a generic FastCache<TKey, TValue> class built on ConcurrentDictionary, using Environment.TickCount64 for wrap-free expiration timestamps and a shared, lock-throttled background timer to evict expired entries across all cache instances. It provides atomic GetOrAdd with factory arguments to avoid delegate allocations, indexer-based upserts, and an atomic Remove(key, value) operation. The library is distributed as a NuGet package (Jitbit.FastCache) and includes a BenchmarkDotNet project comparing it to MemoryCache and HybridCache, plus a unit test project covering eviction callbacks and cache semantics.