When Self-Built Blocks Arrive

Question

When a validator builds its own block locally instead of buying one from an MEV-Boost relay, how far into the slot does that block reach the network, and how does its timing compare to relay-delivered blocks?

Background

You can't tell a self-built block from a relay block by looking at it, so we cross-reference every canonical block against the relays: if no monitored relay reports delivering it, we call it locally built. Arrival is the earliest a Xatu sentry saw the block on gossip (seen_slot_start_diff), measured from slot start. Window: all of May 2026, 222,430 mainnet blocks.

Investigation

How Many Blocks Are Self-Built

Registered vs Unregistered Self-Builders

Split self-built blocks by whether the proposer is registered with any relay and two populations fall out. ~80% come from validators registered with relays: MEV-Boost users whose relay path lost the slot, so they shipped local. They land at ~1.45s, right on the relay curve, because they waited the same ~1s for bids first (the min-bid fallback). The other ~20%, with no relay registration, have nothing to wait for and publish almost immediately, at ~0.51s.

Same split, cumulative: the not-registered group is mostly on the network by 1s, while registered self-builders track the relay curve.

Method: a validator counts as registered if it appears in any relay's registration feed over the year to June 2026 (99.3% of relay proposers do, as they must). Capture is imperfect, so a few "not registered" blocks may be misclassified, which only narrows the gap.

Who Self-Builds, And When

Self-built timing also varies a lot by operator, from ~0.26s to ~3.0s. ether.fi ships self-built blocks at ~0.4s; coinbase and kiln drag up near 2.5 to 3.0s.

Takeaways

• Only ~6.7% of canonical mainnet blocks are locally built; the rest come via relays.
• ~80% of those self-built blocks are from MEV-Boost validators that fell back to local after the relay lost; they land at ~1.45s (the relay median), having waited the same ~1s for bids. The other ~20%, not registered with any relay, ship at ~0.51s.
• Self-built timing is highly operator-dependent, from ~0.26s to ~3.0s.