Running this in the void loop() may trigger a form of governor in Blynk that prevents overt server spamming… as you are trying to process that function thousands of times a second… or even if doing it the strange way you are, it may cause other issue?? hard to say as I do NOT run code that way.
The proper way is to use BlynkTimer and timed functions with reasonable intervals for needed rates.
As for will Blynk run fast… yes, yes it can… see here…
With Cloud it WILL depend on internet latency, but with Local Server then generally only your own network latency matters.