RPM gauge lagging

[skool]

My E30's RPM gauge has a problem. It is always lagging. What might the cause of this problem be?
Thanks.

I took a video of it and here is the link.
RPM Gauge Lag

 

[epj3]

with no corresponding audio, It looks fine to me.

 

[skool]

it lags very much. especially after 3000 RPM.