we will not call this an input lag.
but we can explain this to you.
currently, we are using a stick's channel to send mouse data to SWITCH, but the MAX speed of a stick is limited.(MAX=5, for Splatoon 2 )
for your current settings(12000DPI,1000HZ, HIP=20)
your turning speed has reached MAX even if you move mouse slowly.
when you swipe too fast, your mouse will reach the limitation very soon, but the hold time is much lesser than slow movement.
so you will get a smaller turning when you do that.
you have 2 options:
1. set your DPI and HIP to a smaller one, so your turning speed will not reach MAX, while moving mouse slowly.
for example, DPI=8000, HIP=10, or smaller ones
2. or you can adjust your mouse skills a bit, and use a gentle way to move your mouse.
please do all the test with the latest version V3.12.1.
(Delete old Splatoon 2's config,and add a new one.)