Eric:

Thank you for trying out the code. I hope you saw the note that this first version did not have a proper dealing with 24 Hour format. Try the following with final function set with listable attributes. Also feel free to rename the functions for better readability.

Clear[osplit, opm, pmto24, ldo, dlist];
dlist = Table[
   Block[{$DateStringFormat = {"Month", "/", "Day", "/", "Year", " ", 
       "Hour12", ":", "Minute", ":", "Second", " ", "AMPM"}}, 
    DatePlus[DateString[], RandomReal[{-1000, 0}]]], {10000}];
osplit[x_] := Module[{}, StringSplit[x, {"/", " ", ":"}] ];
pmto24[z_] := 
  Module[{}, 
   If[Last[z] == "PM"  &&  First[z]  =!= "12", 
    ToExpression[First[z]] + 12 , 
    If[ First[z] == "12" && Last[z] == "AM", 0, 
     ToExpression[First[z]]]]];
opm[y_] := Module[{}, 
   ToExpression[
    Permute[Most[ReplacePart[y, 4 -> pmto24[{y[[4]], Last[y]}]]], 
     Cycles[{{3, 1, 2}}]]]];
ldo[s_String] := DateObject[opm[osplit[s]]] 
SetAttributes[ldo, Listable]
ldo@dlist // AbsoluteTiming

If the listable version is slower than you can use style of the previous post to just using Map. Putting the string into a DateList-like format helps but it is not all the source of timing issues. I use a Permute function to swap out the year from 3rd to 1st position but there are other functions that could be faster.

Eric: Please try the following:

Clear[osplit, opm, dlist];
dlist = Table[
   Block[{$DateStringFormat = {"Month", "/", "Day", "/", "Year", " ", 
       "Hour12", ":", "Minute", ":", "Second", " ", "AMPM"}}, 
    DatePlus[DateString[], RandomReal[{-1000, 0}]]], {10000}];
osplit[x_] := Module[{}, StringSplit[x, {"/", " ", ":"}] ];
opm[y_] := Module[{}, 
   ToExpression[
    If[Last[y] == "PM", 
     Permute[Most[ReplacePart[y, 4 -> ToExpression[y[[4]]] + 12]], 
      Cycles[{{3, 1, 2}}]], Permute[Most[y], Cycles[{{3, 1, 2}}]]]]];

Basically generating 10,000 samples that match your format with "AM" and "PM". Then subsequent function that operate on that string format whose aim is to get the list into WL DateList-like format {yyyy, mm, dd, hh, mm, ss} .

DateObject /@ 
  Map[opm[#] &, Map[osplit[#] &, dlist]]  // AbsoluteTiming
(* {0.854198,{Fri 7 Dec 2018 09:40:06GMT-5., ...*)

Functions can be improved and combined to get even more efficiencies. Will Block or With be faster than Module

Please note the AM, PM logic is simplified to just add 12 if PM but it is a bit more complicated than that. But tweaking that should not add too much more time

Eric,

What is the format of your timestamp?

Also, how many are you importing? I tried this on 5000 values and its fairly fast:

In[105]:= bar = Table[DateString[], 5000];

In[110]:= AbsoluteTiming[DateObject /@ bar;]

Out[110]= {1.34714, Null}

Regards,

Neil