A lot (if not most) of the things related to IB and MT4/5 in the manual are not (or no longer) true.
MT4/5 have low memory footprint, are not bandwith-hungry, are fast and stable.
However, MT4/5 in itself were created "for the brokers" allowing for the "last look", "spread management", etc..So, that MT4/5 side latency will depend on the broker a lot.
(as well as on proximity to the broker, quality of the network, your hardware, TCP stack tuning, etc)
Zorro will add some 30-50ms latency to a trade. How many of these are the bridge - difficult to say, i'd say very little.
C++ implementation will for sure work faster than lite-C.
MT5 is faster than MT4 and might/will have a higher tick count, but 1)broker feeds between MT4/5 can be different and 2) increased tick count might put more strain on CPU and might actually increase latency, if you are trading many instruments/systems.