Unfortunately I never had good luck with that card (I have three of them) in the F1, although later GitUp firmware apparently improved compatibility with some cards. Since I switched to a 64GB SanDisk Extreme U3 I've not had any slow card errors, either with GitUp or modified firmware.
I guess I was reading the wrong posts when they suggested that card. Thanks for the tip.
I ordered a sandisk so hopefully I have better luck with it.