Maybe the Windows kernel (or whatever they call it)
"microkernel". But most folks (including those at Microsoft) call it a "kernel".
only checks for task IO every 15mS.
Nope. The time quanta is 10 milliseconds on the vast majority of hardware. In order to consume the entire quanta, an application has to make no operating system calls for the entire 10 milliseconds and run at an elevated priority (which is extremely rare). In addition, when an I/O operation completes (like reading from a serial port), the process waiting for the I/O operation gets a priority boost which typically results in that application being bumped to the top of the heap. Couple all that with the fact that most modern Windows computers have multiple processor cores, on a mostly idle computer, the vast majority of the time a process waiting for a serial port read will run immediately when data arrives.
The latency may come from the Java support library, the USB subsystem, or the driver but it does not come from the kernel.