The newPayload backfill changes the runners on March 27
engine_newPayload just gained another three and a half months of history. That is useful, but the new line has a seam in it: on March 27, the regular-node sample jumps from six observers and two execution clients to as many as fifteen observers and four clients.
If you plot "fastest client" straight through that seam, Nethermind appears to improve from 47.5% of observed slots to 94.9% in two days. It did not suddenly get twice as fast. Nine more runners walked onto the track.