If you read the article linked to in the article I linked to above (which is fairly out of date, from 2010):

https://blog.tsunanet.net/2010/11/how-long-does-it-take-to-m...

you will find that on Linux a context switch takes about 30 usec. More recent measurements that take account of the effect of the TLB flush put the range at 10-300usec.

That means that in 2010, on Linux, you could reasonably expect to do at least 30k/sec. In 2021, with realistic audio processing workloads, the range is probably 3-50k/sec.

The 486 is a much lower register count than contemporary processors, which accounts for the faster context switching.

Modern audio processing software on Linux can run with 64 sample buffers, not 1k.

This recent paper on RT linux on RPi/Beagleboard single board systems concludes that on some of these relatively "low power" systems, 95% of latencies are in the 40-60usec range, which is completely adequate for the majority of RTOS tasks (but not all).

https://www.mdpi.com/2073-431X/10/5/64/pdf

>"The majority of Linux kernels’ measurements with PREEMPT_RT-patched kernel show the minimum response latency to be below 50 μs, both in user and kernel space. The maximum worst-case response latency (wcrl) reached 147 μs for RPi3 and 160 μs for BBB in user space, and 67 μs and 76 μs, respectively, in kernel space (average values). Most of the latencies are quite below this maximum (90% and 95%, respectively, for user space and kernel space). In general, it seems that maximal latencies do not often cross these values."

[ ... ]

"As an outcome, Linux kernels patched with PREEMPT_RT on such devices have the ability to run in a deterministic way as long as a latency value of about 160 μs, as an upper bound, is an acceptable safety margin. Such results reconfirm the reliability of such COTS devices running Linux with real-time support and extend their life cycle for the running applications."

This slide presentation offers up very similar numbers with graphs, also on ARM systems (I think):

https://elinux.org/image/d/de/Real_Time_Linux_Scheduling_Per...

This article shows cyclictest, a very minimal scheduling latency tester, getting the following results on an x86_64 system:

"The average average latency (Avg) is 4.875 us and the average maximum latency (Max) is 20.750 us, with the Max latency on 23 us. So, the average latency raises by 1.875 us, while the average maximum raises by 1.875 us, with the maximum latency raised by 2 us."

https://bristot.me/demystifying-the-real-time-linux-latency/

They conclude

> "Maximum observed latency values generally range from a few microseconds on single-CPU systems to 250 microseconds on non-uniform memory access systems, which are acceptable values for a vast range of applications with sub-millisecond timing precision requirements. This way, PREEMPT_RT Linux closely fulfills theoretical fully-preemptive system assumptions that consider atomic scheduling operations with negligible overheads."

I'm not sure where you're getting your current info from, but I'm extremely confident that it's wrong. If I had to guess, you have not kept up with the impact of the PREEMPT_RT patchset on the kernel, nor scheduling improvements in general, but I don't know (obviously).