Depends on how you are defining portable and fast. Since nearly the beginning of clinical MRI (1977 in the USA) there have been full systems available in semi-trailers.

The USA military has also positioned MRI systems in "containers" at field hospitals. They had the full capabilities of the current systems (for your speed question) and could be relocated rather quickly by various transport means.

There have also been niche MRI systems that could be transported in the trunk of a motor car (typically they were used for imaging small joints like the wrist), they were in the USA market in the early 2000's.

More recently there are systems like the Hyperfine machine https://www.hyperfine.io which is intended for the neurological market.

There are quite a few systems.. they're called hybrid MRI. You can view the concept video here : https://assets.hillrom.com/is/content/hillrom/CombiSuite_Workflow.

There's also another type where the MRI gantry itself moves into the operation theatre.

The type of "hybrid" suite in the animation was first done at by Philips at UCSF around 2000. There is no real portability advantage. The Philips system did allow interactive imaging so you could observe a biospy needle as it was being placed. I forget what the frame rate was however. I suspect at least 16 frames per second.

The question was asked in the favor of needs of surgeons for fast and efficient imaging tools in their operating rooms.

Such devices could help surgeons in many operations such as robotic joint surgeries with very small cuts, robotic work of the heart with very small cuts and etc.