@cgohlke compared the statement df2 = pd.read_csv(csv) on Python 3.7.0a3 and a4 in the Visual Studio profiler. The culprit is the isdigit function called in the parsers extension module. On 3.7.0a3 the function is fast at ~8% of samples. On 3.7.0a4 the function is slow at ~64% samples because it calls the _isdigit_l function, which seems to update and restore the locale in the current thread every time...

3.7.0a3:
Function Name	Inclusive Samples	Exclusive Samples	Inclusive Samples %	Exclusive Samples %	Module Name
 + [parsers.cp37-win_amd64.pyd]	705	347	28.52%	14.04%	parsers.cp37-win_amd64.pyd
   isdigit	207	207	8.37%	8.37%	ucrtbase.dll
 - _errno	105	39	4.25%	1.58%	ucrtbase.dll
   toupper	24	24	0.97%	0.97%	ucrtbase.dll
   isspace	21	21	0.85%	0.85%	ucrtbase.dll
   [python37.dll]	1	1	0.04%	0.04%	python37.dll
3.7.0a4:
Function Name	Inclusive Samples	Exclusive Samples	Inclusive Samples %	Exclusive Samples %	Module Name
 + [parsers.cp37-win_amd64.pyd]	8,613	478	83.04%	4.61%	parsers.cp37-win_amd64.pyd
 + isdigit	6,642	208	64.04%	2.01%	ucrtbase.dll
 + _isdigit_l	6,434	245	62.03%	2.36%	ucrtbase.dll
 + _LocaleUpdate::_LocaleUpdate	5,806	947	55.98%	9.13%	ucrtbase.dll
 + __acrt_getptd	2,121	1,031	20.45%	9.94%	ucrtbase.dll
   FlsGetValue	647	647	6.24%	6.24%	KernelBase.dll
 - RtlSetLastWin32Error	296	235	2.85%	2.27%	ntdll.dll
   _guard_dispatch_icall_nop	101	101	0.97%	0.97%	ucrtbase.dll
   GetLastError	46	46	0.44%	0.44%	KernelBase.dll
 + __acrt_update_multibyte_info	1,475	246	14.22%	2.37%	ucrtbase.dll
 - __crt_state_management::get_current_state_index	1,229	513	11.85%	4.95%	ucrtbase.dll
 + __acrt_update_locale_info	1,263	235	12.18%	2.27%	ucrtbase.dll
 - __crt_state_management::get_current_state_index	1,028	429	9.91%	4.14%	ucrtbase.dll
   _ischartype_l	383	383	3.69%	3.69%	ucrtbase.dll