Anyone notice significant speed improvements to the UI after move to OCI??

No, often the contrary but I think mileage may vary depending on your provisioning tier. I am sure at the lowest level (shared tier) you would see improvements across the board in some cases as its a more modern setup with better global coverage. Note that you need to track over a longer period of time and across your account (vertically and horizontally) for it to make any sense to definitively make statements about performance.

No.