hahahah.. no. just, no. you set up the machine to work as a cnc control, not a web surfing box. you turn everything off. ive never seen any for of "windows" issue running mach3.
timing might be better, hard to say. mach does not have the best timing (hence the misses slow pulses i mentioned), but i dont think thats parport related, or mach related. linuxcnc has much better pulse timing than mach3, and its also just software.
you said grbl. that implies arduino level stuff. theres plenty of higher power systems out there for sure, but they dont cost $9.99 on ebay
also theres a reason ethernet and usb are not used much and parport is. parport works. sure its slow and limited, but it works. ethernet and especially usb are inherently not realtime. so called realtime ethernet system like my beckhohf ethercat system are not really ethernet as you know it. they use the host tranciever and cable, but its all proprietary protocols. you cant stick your modem or printer on it