There are three speed limits:

• camera to RAM write speed - this is what you experience with fast shutters and no flash, at the beginning of taking photos,
• RAM to Flash bridge write speed - this is the limitation of the MCU/bridge/hardware elements,
• Flash card write speed itself - what the top speed of the card is.

It is easy to test whether you are at the limit of your camera.

• Take uncompressed RAW images of the same topic (from a tripod, manual focusing, or even better: just put the lens cap on).
• Start shooting a sequence, then the camera gets slow, just keep shooting 20 more.
• Measure how long did it take to capture the last 20 images (the more image you take, the more precise your calculation will be). => TOTAL TIME
• Now take the photos, and add their size (the real size in bytes, not the allocated size). => TOTAL SIZE

And if you divide TOTAL SIZE with TOTAL TIME, you will have the worst of the data transfer rate of the RAM-to-Flash and the Flash card. If your card is rated faster than that, you have a fast-enough card. If your card has about the same speed, then either the flash card is limiting the speed OR you are at top speed of the RAM-to-Flash path. Borrow a faster Flash card, do the same trick, and you will see which one.

One note: there are quite a few fake Flash card out there, basically telling you a much better data rate than they actually support. You can test this with a high speed card reader (USB3), just move a 500 MByte or so to the card (in one chunk), and measure the time. Again, divide 500 MByte with the elapsed time, and you will have your write rate. If it matches the number on the card, cool! :-)