Wesbite super slow on iphone / very fast on desktop with same connection on wifi

I'm having an issue on my website.

The speed is perfect on desktop - really fast.

However - it is really really slow in Iphone/Ipad.

THe thing is im using the same connexion wifi - soit's not related to the 3G/4G connextion.

( tried in different place e- also tried in 3G / 4G ).

Do you have any idea why is this happening ?

I'm using W3 Total Cache and it doenst change anything .. .

Hope to have some highlite , thank you your time guys !