So bitcoin block generation speed is 10 minutes, is this minimum transaction speed?
Then how can faster it?
I find at 'chainparams.cpp', consensus.nPowTargetSpacing = 10 * 60;
So this is 10 minutes, so if I change it to 0.1 * 60, then transaction speed also become 6 seconds?
What else parameters should I change to speed up?
The real answer is you you shouldn't speed it up.
10 minutes is an AVERAGE block time, but in theory it can be much faster for 2 weeks if you put in lots and lots of additional hash power into network. However this time is there to make sure network is stable and robust, you shouldn't aim to speed it up. There are plenty of solutions offering instant bitcoin transactions and LN will be instant settlement.