# Compiler Startup Performance

Compiler startup performance is a key factor affecting happiness of F# users. If the compiler took 10sec to start up, then far fewer people would use F#.

On all platforms, the following factors affect startup performance:

• Time to load compiler binaries. This depends on the size of the generated binaries, whether they are pre-compiled (for example, using NGEN or CrossGen), and the way the .NET implementation loads them.
• Time to open referenced assemblies (for example, mscorlib.dll, FSharp.Core.dll) and analyze them for the types and namespaces defined. This depends particularly on whether this is correctly done in an on-demand way.

• Time to process "open" declarations at the top of each file. Processing these declarations have been observed to take time in some cases of F# compilation.
• Factors specific to the specific files being compiled.

On Windows, the compiler delivered with Visual Studio currently uses NGEN to pre-compile fsc, fsi, and some assemblies used in Visual Studio tooling. For .NET Core, the CrossGen tool is used to accomplish the same thing. Visual Studio will use delayed NGEN, meaning that it does not run NGEN on every binary up front. This means that F# compilation through Visual Studio may be slow for a few times before it gets NGENed.