Some possibilities:
1. If you've used the card for a long time without formatting, the free space may have got fragmented. This causes read/write speed to reduce.
Solution: reformat the card.
Due to the nature of solid state memory, "fragmentation" has little to no impact on SD cards read/write performance. It doesn't matter if the next block of data is sequential or not, every block has the same access time. Also, due to wear leveling and multiple memory banks, data is almost always fragmented.
Once the capacity of a SD card has been written, it must start performing what's known as "Garbage Collection" to free up blocks. This remains true even if the files have been deleted or a conventional fast format has been done. Garbage Collection only degrades write performance and has no impact on read speed. All modern SSD uses a process called "Trim" to avoid this issue. But, SD cards don't use Trim. The only way to restore original out of the box write performance is to reset all the memory blocks. Canon cameras offer a low level format option that will do this as can the SD Card Association format utility. A normal format simply resets the FAT data.
If read speed of a card suddenly degrades, it could signal some sort of failure. Read errors are a common occurrence and are usually immediately corrected. But if the correction code is unable to fix the error, the controller will perform multiple reads until it's valid. If the read errors reach some threshold, the data is moved to another block and the failing block is marked as bad. It's all supposed happen in the background. But sometimes it becomes bad enough to where the user notices performance issues.
Performing a in-camera format won't hurt anything. But, I'd suggest trying another card. If that fixes the problem, consider retiring the problematic card.
