How much (estimated percentage) is going to worsen the response-time of the database if the logging level is lowered to "DEBUG". ?
I work at VoltDB. I wouldn't recommend setting the overall logging level to DEBUG except perhaps for a DEV environment for limited tests. It's hard to say what % the performance impact would be. Everything might be a bit slower, but there could be a lot more GC triggered which can have an unpredictable impact.
You can adjust the level for specific loggers, if you have an idea of the area of functionality where you want to see more detail. We have a howto guide on configuring logging here: https://github.com/VoltDB/voltdb/blob/master/examples/HOWTOs/ConfigureLogging.md