Indeed. I'm not sure what our next steps are yet. Once 1.13 gets released we'll wait a few weeks to ensure stability and compatibility with the stable (non-beta) releases of the server and plugins (e.g. just checking comments to see if others have issues). So, we'll likely discuss what plugins we want to carry over rather than do more testing. However, we still want to get a new world benchmark with our plugins, so we may launch a new world with the same seed for an hour just to see if we get the same TPS issues.
As far as LagMeter is concerned, I'm not sure what caused the discrepancy. I want to say LagMeter does a rolling average over 30 seconds to a minute, as the TPS was at .53 or something about a minute after the server started while Essentials listed it at 20 or 17. LagMeter wasn't available right away (something about the PollingPeriod) while Essentials was available immediately. (whatever it was at the time), and LagMeter went up as time progressed. I think it was at 20 TPS within 2 minutes? Maybe 3?
We got to about 13-14 TPS with 5 players online with Group 1 & 2 plugins removed as well as Towny + WorldGuard and 12-13 TPS with 3 players in their same locations as the prior test, but with Towny + WorldGuard included. So, if we do more testing we'll likely focus in those regions to see if it's one particular area. The mods definitely consider next steps and we'll go from there.