I think you are mis-understanding the difference between 'SD' and 'SDHC'. It's not a speed designation, it's a capacity designation.
The original 'SD' cards had a limit of up to 2 GB capacity. This was due to the fact that they used the FAT16 file system.
'SDHC' cards are newer cards with capacities from 2 GB to 32 GB. This is because they use the higher capacity FAT32 file system.
An even newer designation 'SDXC' is being introduced for cards from over 32 GB and up to 2 TB. These new cards will be using the exFAT file system.
The original 'SD' cards had a limit of up to 2 GB capacity. This was due to the fact that they used the FAT16 file system.
'SDHC' cards are newer cards with capacities from 2 GB to 32 GB. This is because they use the higher capacity FAT32 file system.
An even newer designation 'SDXC' is being introduced for cards from over 32 GB and up to 2 TB. These new cards will be using the exFAT file system.