The advertisement speeds aren't average speeds when filling the card continuously. They are some top speeds.
For fast writing you should use identical cards and never delete files. Always format.
Rest of this post is some technobabble and trying not to go of abyss of flash memories of filesystems.
Let's tackle the always format first. When using FAT32 filesystem, there is file allocation table (FAT) which says this file is in these clusters. Best scenario is that file is one chunk of sequential clusters. When writing image to card the camera can first write to FAT that there will be image on these clusters and then write it. The behaviour is then single write to the FAT address and sequential write starting from address which the allocated clusters are until otherwise told. Speed difference is about ten times between single writing and same amount of sequential writing.
When deleting files the disk space is freed in FAT. The data is still there and flash memory can't be rewritten unless erased first. When rewriting, camera and card does a lot of magic and it takes time to get the data on disk. Formatting erases the card fully and gives possibility to just write.
I don't have experience on the exFAT filesystem but someone who wrote driver for it said, it's like FAT beaten to beyond recognition and then put back together to keep patent royalties continues to flow....
There are many possibilities when SD-card can ask for delay. So using both cards identical the timing model is same and camera may or may not prepare for it. Also larger cards usually are faster on small writes like few MBs of JPG. I believe this is result of the addressing model of the flash chip inside the card. So far I haven't seen any processor which has two independent SD hardware, but I've multiplexed one to many cards and then it is just small matter of programming. So when both cards are equally fast, the writing is fast.
This stuff is usually hidden in many layers of abstractions. On computer there is card reader, maybe USB, all other HW and OS with its buffers and delays, but on embedded world this comes important.
Arduino sdfatlib provides some performance data:
sdfatlib/CardPerformance.txt at master · jbeynon/sdfatlib · GitHub which is using SPI interface, not SD interface to communicate with the card but same rules apply.
My favourite brands are by the way Sandisk, Lexar and newest Trancends. I like how they behave.