I have rarely seen speed affect tension. What you described is a complete computerized LA set up. If speed affected tension those computers would most certainly be programmed to account for that but I really doubt it is the speed of the machine.
If she is setting the machine and walking away any number of things could be happening.
Are you requesting all cotton thread? I have had a lint fluff get stuck in the bobbin spring unbeknownst to me and cause my tension to be off. It could be fine on my tests and look fine from the top but I don't realize I had a problem until the quilt is advanced and I see the back. Or I go to tie off and realize my bobbin thread has a lot of drag and doesn't want to come up easily. Often those lint fluffs will dislodge on their own so the tension problem will come and go. I have never had this happen when using poly thread top and bottom. So if you really like this quilter and don't want to change and she has no clue what is causing the problem then I would suggest you request only using polyester thread in top and bobbin. A good name brand one at that. Filtec Glide has never ever given me a problem like this. Could also be the magnaglide bobbins. When I have a magna glide in I never have tension issues.