yes I was adding the acid gradually so as not to overshoot. Also I was not sure at what temperature the reaction of the phosphates in malt with the Ca in water occur. I later added more lactic acid for a grand total of 7 ml to get to 5.3. It later went up to 5.5 then finally settled out at 5.4. I eventually will get a 2 decimal point pH meter so I can have more confidence in knowing a 0.1 change detected is real.
I use brew in a bag and bob the bag up and down to stir the wort so my thermocouple reads temp well. I more or less do this continuously when not measuring stuff. I also do at least two bag lifts and let it drain for one minute during each step then punch the bag back down open it and stir up the grain well so its wet like a slurry(5.5gal water in mash 9.75 lbmalt). At mashout I drain the bag well in a turkey fryer insert held up with inverted bowls in large container. I then slowly and intermittently rinse the grist for 3 batches(170F 1.5 gal total 3 rinses). I generally add the first two rinses to the boil but the third rinse I boil in a separate pot. If needed, I add the third rinse at end of boil. OG 1.06 vs 1.047 recipe called for in this case.
The reaction rate between an acid and base like HCl and NaOH is one of the fastest chemical reactions. However, yes the gazillion other reactions effecting (or is it affecting?) pH may take time.