Speeding up StringFromList on a lengthy string

// This function processes a string (read buffer in this case) and 
//      returns a wave of floating numbers
 
 
Function   Wave_from_StringFromList ( sample_str )
    String sample_str // A semicolon-separated string list
    
    String separator = ","
    Variable separatorLen = strlen(separator)
    Variable numItems = ItemsInList( sample_str , ",")
    Variable offset = 0
    Variable i
 
    print "\t number of items in the list : ", numItems
    
    Variable timerRefNum
    variable microSeconds
    timerRefNum = StartMSTimer
    
    make /FREE /d /n=(numItems) temp
    string item
        
    for(i=0; i<numItems; i+=1)
        // When using offset, the index parameter is always 0
        item = StringFromList(0,  sample_str  , separator, offset)
 
        temp [i] =  str2num(item)
    
        offset += strlen(item) + separatorLen
        
    endfor
    
    microSeconds = StopMSTimer(timerRefNum)
    Print /d microSeconds  /1e6 , " seconds "   
        
    duplicate /o temp out 
    
End
 
////////////////////////////////////////////////////////////////////////

// output when running the above function
Wave_from_StringFromList ( root:RTO_str )
     number of items in the list :   61954
  22.6564711   seconds