It's terribly slow unless you don't care about encryption: https://github.com/weaveworks/weave/issues/1925

The per-packet processing overhead is a real and unresolved problem.

I'm using Rancher's networking now instead. It uses ipsec between hosts, so everything gets handled by code paths which have been optimized in the kernel, and performance is good (especially if you have a not-ancient CPU and have the AES-NI instruction - then wirespeed gigabit works with acceptable overhead).

I like Weave's decentralized architecture and wish it were realistic to use it.

If they are going to stick with a user-space solution they probably need to use DPDK or one of the other high-performance software defined networking toolkits, which tend to process packets using a SIMD approach.