How to fix low sensitivity mouse delay?

Question

In any FPS/TPS game, low sensitivity results in a substantial 'input delay', ~300ms, for large swings. That is, if I swipe quickly to make a 180-turn, the camera's still turning (post-swipe).

I've ruled out some possibilities, listed below. All considered, what could be the problem, and how do I fix it? Help's appreciated.

---

Causes Considered:

• Mouse (hardware); changed several mice, w/ 3 brands, wired & wireless
• Mouse (settings); tweaked Pointer Speed, DPI (hardware), Enhanced Pointer Precision
• Specs; GTX 1070 6GB, i7-7700HQ, 12GB RAM, Win-10.0.17134 x64, free disk space
• Game limitation; Fortnite streamers freely 180-flick - my crosshair drags cement
• Viruses/malware; none that Avast & Malwarebytes detect