Leatrix Latency fix disables nagles algorithm and adjust the ACK Frequency. Basically it makes your computer send all of its out going packets as soon as possible instead of trying to combine them into more effecient packets. When it's on your packets sit in a buffer within a certain window of time and it optimizes them in that window of time resulting in higher throughput with a fixed pre latency (e.g. it's waiting, so your latency is higher).

Turning it off gets rid of that wait, thus your latency goes down to what it should be. But at the cost of effeciency. You are likely to see a 10% to 50% drop in overall bandwith by turning it off. A 50 meg connection will drop to 30 something on everage and a 25 meg connection will drop to in the 10's to -high teens 10-19 mbps etc (depending on traffic type etc).

The problem is that it affects your upload bandwith too. And on most internet connection Your upload bandwith is a lot lower than your download bandwith.

E.g. My connection is 50 mbps down and 6.5 mbps up.

On connections with low Up streams (e.g. dsl with 1 meg up or lower) or a low upstream in general, Turning off nagles algorithm can do more harm than good by causing your PC to choke on bandwith and then you see REAL Lag spikes, as in it's literally throttiling outgoing server data and you lag like hell.

Based on my observations I would say that in order to use Nagles on a computer, you should meet the following requirements

Available Upload Bandwith With Nagles off | 5mbps
Available Downsload bandwith with nagles off | 10mbps

That's not counting what other programs you have running etc. This game needs at least 2 meg up and 5 meg down to be stable when it get's bursty.