So bitcoin block generation speed is 10 minutes, is this minimum transaction speed?
No, that is the average block time that we want.
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?
If by transaction speed you mean confirmation time, then changing that will effect confirmation time because it effects the block time. Note that changing that does not guarantee that blocks will be found every 6 seconds, it only says that the average time in between blocks is 6 seconds.
What else parameters should I change to speed up?
You will need to change the maximum proof of work target (which thus decreases the minimum difficulty). The proof of work target is what actually regulates block times.
Thanks.