Does an iPad case like my Zagg Folio (plastic) have any effect on wi-fi reception speeds? T.I.A.

Is it possible? Technically yes, if it met some very interesting design constraints.

Is it likely? No. Plastic at that thickness doesn't attenuate the signal.

It shouldn't have much effect, if any. There will be some degradation of the signal, since any radio wave will lose power going through a solid medium, but it would be such a tiny amount that I doubt the effect could be measured.

Lets face it, your wifi signal is already travelling through walls etc. Right now, mine is travelling through a double thickness wooden wall, an insulated steel exterior wall and a truck window. The router (a not very new Linksys unit) is around 100 yards from where I'm sitting, and I have a good signal.