What is TTH?
Tiger Tree Hashing or TTH is used to verify the integrity of large chunks of data.
The data is split into small pieces which are individually hashed, then hashed together until one,
root hash is formed. A TTH has looks something like: 4NIABZVGR4HOTNRPOMH2IRIJQSJYKFHEEIQUJWY.
Detailed information can be found here. DC++ 0.400 introduced mandatory TTH.
Once DC++ hashes all of your share (yes this will take a while) it will only hash new files.
The hashing thread in DC++ is set to low priority, so it shouldn't interfere too badly.