Blocks can be profiled using pprof and thanks to the crate cpuprofiler.
In order to profile a block, the project must be compiled in debug mode with the profiling feature enabled:
cargo build --features profiling
In order to profile a given block e.g. load block:
./target/debug/i3status-rs --profile load --profile-runs 10000 <config>
where <config> is a path to a toml config file.
It will generate a load.profile which can be visualized using pprof.