It's the hundreds of blocks they are finding is bogging them down.
{
"blocks" : 4187,
"currentblocksize" : 1000,
"currentblocktx" : 0,
"difficulty" : 0.00097655,
"errors" : "",
"generate" : true,
"genproclimit" : 7,
"hashespersec" : 6282,
"networkhashps" : 4376681,
"pooledtx" : 0,
"testnet" : false
}