Heroes 5.5 WarGame Edition - Tactical Edition.

Adds lag, which isn't ideal given the mod's concept.

That's true, but following that logic, battles also "add lag" due to reduced speeds, albeit to benefit tactics. The question is what's more important. Plus, consider that the length of a hero's turn will be significantly increased.

However, Dirman writes to me in ICQ that technically, it's impossible to do this – to nullify or reduce a turn when heroes meet, because the event isn't triggered (unlike meeting neutral creatures). If that's the case, the concept will have to be revised anyway.