Introduced a bottle, baby now fussing at breast

[MsFiremanSam]

DD is 6 months and has been ebf. We have just introduced 1 bottle of formula in the evenings, alongside starting weaning. She really seems to love her bottle -as soon as she sees it she flaps about excitedly and tried to grab it, then downs 6-7oz. She also seems a lot more content in the evenings, whereas before she would fuss on and off the breast for ages.
Problem is that now when she bf she seems to get much fussier, and I have to keep alternating boobs, whereas before she'd be happy with just one. Could she be frustrated that it's not as fast as a bottle?
Ideally want to introduce another bottle in a few weeks, but bf until she's sleeping through.

Mixed feeder here, to a 6mo.
We use a supplemental nursing system to give formula, but same principle - of making them work for it - could be done with a bottle. Hold the bottle at a shallower angle and make them take breaks every oz or so. I was told by a bf counsellor to aim for the bottle feed to take as long as the breast.
This may help or it may be something completely different.