High mempool load causes block proposal delays

[riptl]

A major mining pool using this implementation has reported delays in distribution of block templates to miners with large amounts of pending transactions. No delays seem to appear when the mempool is empty.

• [ ] Reproduce a significant delay of block production (>1s) in a test environment or on main net
• [ ] Profile block template generation code path
• [ ] Optimize code or mitigate impact with configurable limits