nm, got it.
Batting.csv and Fielding.csv lack carriage return characters, having only linefeeds. Some users systems presumably have setups or other software installed that allows them to cope with that; others don't.
For anyone who wants to try fixing that to get a speedy Lahman import and isn't afraid to run vbscript given by some bloke on the internet

- copy and paste the following into Notepad:
set oFS = createobject("scripting.filesystemobject")
set oFILE1 = oFS.GetFile("Batting.csv")
oFILE1.Name = "Batting.bak"
set oFILE2 = oFS.GetFile("Fielding.csv")
oFILE2.Name = "Fielding.bak"
set oTR1 = oFS.OpenTextFile("Batting.bak")
set oTW1 = oFS.CreateTextFile("Batting.csv")
set oTR2 = oFS.OpenTextFile("Fielding.bak")
set oTW2 = oFS.CreateTextFile("Fielding.csv")
Do
strTemp=oTR1.ReadLine
oTW1.WriteLine(strTemp)
Loop Until oTR1.AtEndOfStream
Do
strTemp=oTR2.ReadLine
oTW2.WriteLine(strTemp)
Loop Until oTR2.AtEndOfStream
set oFILE1 = Nothing
set oFILE2 = Nothing
set oTR1 = Nothing
set oTR2 = Nothing
set oTW1 = Nothing
set oTW2 = Nothing
set oFS = Nothing
Msgbox "Finished"
Save this as "whatever.vbs", in the same folder as your Lahman database. Double click the file.
That creates versions of the Batting.csv and Fielding.csv with proper end-of-line CR/LF combos; if that is the reason for slow loading/failure to load, this should fix it. (Temporary fix, obviously, for those who want to try). (All it does is read the line and writes it back to a new file, but doing it with vbscript means it gets a vbCrLf end of line combo).