Zooko has commented here about this subject before, see this post.
To expand a bit: some optimizations we’ve proposed impact our security proof and assumptions in non-trivial ways and require expensive and time-consuming analysis. We’ve considered a lot of potential changes and approaches, and I personally believe we have enough available to us after launch to gradually make performance better without impacting the design much.
This isn’t to say we aren’t doing any optimizations before launch; we’re currently working to improve performance using novel constructions and have contacted additional experts to verify their security before we commit to them. We’re prioritizing optimizations based on their impact to the scheme’s security, their practicality and their impact. In some cases we may be making changes to the scheme before launch which do impact our security proof but simultaneously improve the design and performance.
We’re all very aware of Bitcoin’s scalepocalypse and keep that in mind in every decision we make. 