[fulu] Inefficient network processor in fusaka-devnet-3

### Describe the bug

network processor "cannot accept work" is high in `fusaka-devnet-3` super nodes

this is on a 1k hoodi node (which does not subscribe to all subnets)

"regen busy" is the main reason

this could lead to higher wait time of jobs, especially `beacon_attestation`, it could be ~1.7s

### Expected behavior

"cannot accept work" should be close to 0, or need to investigate the reason if not

### Steps to reproduce

_No response_

### Additional context

_No response_

### Operating system

Linux

### Lodestar version or commit hash

unstable

Start by locating the network processor and the metrics behind "cannot accept work" and "regen busy" in the Lodestar codebase. Reproduce or inspect this behavior on a fusaka-devnet-3 1k Hoodi node, then determine why work is rejected and verify that beacon_attestation wait time and rejected-work metrics improve.

Written by the indexing model from the issue text.