Here is a simple pure python example:

digits = ''.join([str(i) for i in range(10)]*10000000)
%timeit digits.isdigit() # --> 2X+ slower on python 3.7.1

Basically in Pandas C-code parser we call the isdigit() function for each number that is to be parsed. so 12345.6789 calls isdigt() 9 times to determine if this is a digit character that can be converted to a float. The problem is in the latest version of Python with locale updates isdigit() takes a locale argument that seems to be passed over and over slowing down this check. Is it possible that we disable any local passing from Python down to lower-level C code, or simply set the default locale to 'C' to keep it from thrashing?