The DLR is Microsoft's Dynamic Language Runtime, and it consists of a set of services that run on top of the CLR and are used to implement dynamic languages on the .NET Framework.
Regarding overall performance of IronJS, Holmstrom said:
"All in all I'm happy with the performance of IronJS, considering that I have not spent a single second optimizing it and have made conscious decisions to not use several DLR optimization techniques to keep the code as simple as possible up until the 0.1 release."
Microsoft has hired expertise to help build up its dynamic language support. The company hired Jim Hugunin, a Python expert, to bring his IronPython project in-house, and Microsoft later hired Ruby expert John Lam to focus on the IronRuby work. Lam recently left the team to work on a new project at Microsoft.