I found a solution and I'm not sure If this would work for everyone, but If you are on Speed 3 and then press F2 for "Buy Mode", it's normal.
I've stumbled across a temporary workaround - in Buy/Build mode, if I open the Gallery (F4) and then immediately close it, the lag seems to stop until I switch Modes again.
This is very similar to opening the three dots menus (such as "Read..." on a bookshelf) in Live mode to make the lag go away.
It comes back as soon as you switch modes, but it's a temporary way to get around the lag until a proper fix is developed.
Hope this helps - sorry if it was already posted - quite a few pages of comments to read through.
