[ethereum/go-ethereum] Sudden drop in TPS after total 14k transactions (#17447)

Hello geth team, perhaps you have ideas where this sudden drop in TPS in quorum might come from:
https://github.com/jpmorganchase/quorum/issues/479#issuecomment-413603316

#### System information

Geth version: quorum Geth/v1.7.2-stable-3f1817ea/linux-amd64/go1.10.1
OS & Version: Linux

#### Expected behaviour
constant TPS

#### Actual behaviour
For the first 14k transactions, we see averages of ~465 TPS, but then it suddenly goes down considerably, to an average of ~270 TPS.

#### Steps to reproduce the behaviour
See https://gitlab.com/electronDLT/chainhammer/blob/master/quorum-IBFT.md