From what I recall the completely fair scheduler (CFS) used by default on most Linux systems has a lower average latency than the RT kernel. The RT kernel just gives you more consistency, hence the CFS having lower latency “on average”
So honestly for opening Firefox it’ll probably depends more on your SSD data rate, but in theory it’ll open faster on a “regular” distribution most of the time.
Real time is good for things like audio processing where having better guarantees that a process will get its share of the CPU is a benefit.
I have to replace a garbage disposal later this week. I’m not looking forward to it, but hopefully it’s an easy project.