Would you consider lactate production to be promoter or inhibitor of acidosis?

As you probably know, enzymes such as lactate dehydrogenase allow you to reconvert lactate into pyruvate in the heart. Lactate can also be used as fuel in the skeletal muscles, and the positive ions produced by the breakdown of lactic acid (lactate being the negative ion) are not thought to be the ones that inhibit muscle movement anyway. So lactate isn't your enemy; its presence in the blood to varying degrees is only a barometer which gives a pretty good (though not perfect) indication of intensity (as does ventilation).

Lactate transporters (aka monocarboxylate transporters, or MCTs) are seen as instrumental in removing lactate from glycolytic muscle fibers and into oxidative muscle fibers with more mitochondria. There are 8 known "isoforms" of MCTs, designated MCT-1 through MCT-8, with the MCT-1 and MCT-4 isoforms now believed to be the principal transporters responsible for lactate transport in muscles. The MCT-1 and MCT-4 isoforms are both fiber-dependent and pH-dependent, so muscle fiber composition may have a bearing on which workout speeds benefit a particular athlete most; also, muscle buffer capacity plays a role in facilitating lactate transport, and some evidence suggests induced alkalosis assists training in boosting MCT-4 content. Testosterone and thyroid hormone T3 will also elevate MCT-4 content and increase lactate transport.

Specific workout speeds which target MCT-1 content center around the effort intensity between the ventilatory threshold and the respiratory compensation point. A general neighborhood for pace guidelines would be about the speed a runner could sustain for 39-44 minutes in an all-out effort, provided the pace was even throughout. If this pace is run in medium-duration segments with rest periods of one-fourth to one-third the duration of the running periods between each segment, a runner should be able to briefly produce just enough muscle lactate to provide an adequate opportunity to process the lactate without lowering the pH so much as to hinder neural efficiency, let alone "tie up" or have adverse effects on the organelles within muscle cells. Repeated bouts of 3 minutes to 15 minutes, with 25-35 minutes of total time accumulated at the target pace, provide an excellent stimulus for improvement. This is basically an interrupted "hard" 25-35 minute "tempo run" which would be too hard for regular use if the pace was carried as a continuous run rather than breaking it into segments. Segments of 3 minutes are normally too short to achieve the desired feeling for this workout, so a minimum of 7 minutes at a time would be more desirable, although the 3-minute bouts can be used under special circumstances (i.e., at altitude or on difficult terrain or when trying this effort intensity for the first time during a season).

Boosting the MCT-4 content is best achieved via a sudden flooding of the muscles with lactate, which of course requires higher intensity of effort. Being mechanically efficient at the speeds involved is paramount for maximum effectiveness, as is some introductory work of slightly lower intensity, so work your way gradually up to the really intense workouts. Two sets of 2 x 400 at 1-2 seconds faster than 800 race pace with 1 minute walk between reps in each set and 5 minutes walk/jog between sets is an example of a high-intensity workout. There isn't much running involved (aside from the warmup routine), but there doesn't need to be in order to produce enough muscle lactate. Fatigue has peripheral and central neural components as well as being defined by inhibition of muscle movement or by depletion of fuel. Therefore, on a high-intensity workout such as this one, you want to run as fast as you can throughout while still preserving as much neural coordination as possible at the end of the last rep, so doing more reps might look impressive in the logbook, but any possible added benefit is likely outweighed by other (negative) factors.