As the camera theoretically has to make the double amount of writes, is there a performance impact? (especially whilst shooting RAW continuous and filling the buffer).
There almost always is a performance penalty. How significant it is can vary widely from one camera model to the next. The only exception would be if both slots support the same type of card, there are more-or-less identical cards (same brand, model, batch, and current amount of consecutive free space) in both slots, and the same exact files are being written to both cards simultaneously.
If one slot is slower than the other, or the card in one slot is slower than the other, the bus speed of the entire card bus will often be slowed to the (lack of) speed needed to write to the slower slot/card.
At the same time if one card is faster than the other does it wait until the image has been written to both cards before releasing the space in the buffer?
Yes, the space in the buffer will not be released until both files have been written.
So even if you're using a super-fast CF card in one slot to write raw files and a more pedestrian speed SD card in the other slot to write much smaller JPEGs, many cameras will slow the write speed of the raw files to the CF card to the same data rate as the write speed of the JPEGs to the slower SD card.
Camera is 5D4, although I do wonder if there's a difference between the manufacturers?
Not only can there be differences between manufacturers, there are often differences between specific models within the same manufacturer's offerings.
The Canon EOS 5D Mark IV has a CF card slot fully compliant with the UDMA-7 standard. It has an SD card slot fully compliant with the UHS-I standard. According to CameraMemorySpeed.com, the 5D Mark IV peaks out at about:
- 112 MB/s with the fastest CF cards
- 79 MB/s with the fastest UHS-I cards
Another way of looking at it, which CameraMemorySPeed.com also tested, is that the 5D Mark IV can write the following number of (RAW+JPEG/RAW/JPEG) images to the fastest memory cards of each type, used alone, in 30 seconds:
- UDMA-7 CF card: 67 - 99 - 194
- UHS-I SD card: 53 - 74 - 188
This difference is much less dramatic than the 5D Mark III, which is UDMA-7 compliant but not UHS-I compliant, but there is still a significant difference.¹
Then there's the whole issue of using UHS-II cards in UHS-I slots. UHS-I has two supported bus speeds: 50 MB/sec and 104 MB/sec. Some UHS-II cards will revert to 104 MB/sec when used in UHS-I slots (example" Toshiba Exceria Pro UHS-II ), other UHS-II cards revert down to 50 MB/sec when used in a UHS-I slot (example: SanDisk Extreme Pro UHS-II), even if that slot supports the faster 104 MB/sec clock rate.
Canon has been roundly criticized for the SD card slot not supporting UHS-I in many of their upper tier cameras that also included much faster CF card busses, even though UHS-I has been out for a while. If an SD card is used in a dual slot configuration, it also slows down the CF slot. Several of their other cameras with only a single SD slot and no CF slot, some of them older than the dual CF/SD card models that don't support UHS-I, do support UHS-I. Canon currently offers no ILC models that support UHS-II speeds. As of July 2018, a handful of cameras from other manufacturers do support USH-II speeds, but only five of the ten with dual card slots support UHS-II speeds in both slots.
Gavin Farrington did a detailed look at how the same two cards fared in the Canon EOS 5D Mark III with various save options selected.
¹ Rob Galbraith's 'CF/SD/XQD Database' lists the top speed of the 5D Mark III using the fastest tested cards at about 80 MB/s with CF cards and about 20 MB/s with SD cards. The 5D Mark IV's "slow" SD slot, at 79 MB/s is functionally as fast as the 5D Mark III's "fast" CF slot. It's just not as fast as the 5D Mark IV's faster 112 MB/s CF slot with the right cards.