MTU

It depends if you have lots of errors (re transmissions) on your connection.  Larger packets take longer to re-transmit.

A gov't must control the natural phenomena of radio waves because radio waves are essential to its safety and security.  For example, I can't create a doodad to broadcast on an Extremely Low Frequency (3-30 Hz) because I'd interfere with DoD communications with submarines.  Likewise, I shouldn't broadcast on the same frequency as Air Traffic Control and Landing Systems.  This chart is how our gov't controls and allocates our radio waves...or broadcast frequencies...to protect its interests.

You leapt from my mention of 1 or 2 channels to "Internet quality."  Can EMI affect Internet quality?  No.  Can EMI affect 1 or 2 channels (frequencies) on the coaxial cable delivering your Internet service?  Yes.

However, EMI won't interfere through the insulation of the cable itself but could interfere through your manipulation of the cable:  splitters, couplers, nails, metal tape, etc.  From my experience, most of these splitters and couplers are either under furniture or in corners to avoid deflections.  You'd need an EMF setting directly atop a splitter to obstruct 1 of the 32 frequencies of your service.

I don't know what's causing your uncorrectables but it might not even be within your house.