Hi,

I guess the nice values are null and void, IOW IIUC they don't affect
anything [1], at least for cosmetical reasons remove it from your
limits.conf. If you fear that something could become unresponsive, you
could decrease "rtprio 99" to a lower value. I never did it myself, but
some users do so.

To optimize real-time performance you should start with using Rui's
script rtirq [2]. There are other things to improve real-time
capability, often setting the cpu frequency scaling governor to
performance improves a lot [3]. There are additional useful
opportunities, e.g. unbinding USB devices that share an IRQ with the
audio device could be helpful, but a lot of hints provided by the
Internet are plain nonsense. The most important thing to consider is
choosing the appropriate kernel. For some tasks it's useful to build a
rt patched kernel [4]. The so called "lowlatency" kernels provided by
Ubuntu flavours gains you more or less nothing. If you should use a
mainline kernel, simply add "threadirqs" to the boot parameters, for
syslinux it does look like e.g. this