I wonder if your kernel is using more CPU than necessary to keep Reaper operating stable and at as low a latency as possible.
Here's my story:
When choosing a distro, I wanted it to be "not heavy-weight" ("lightweight" distros might be a bit too slim on options, but I didn't want a "bloated" one either or one that used more resources for graphics), it had to be relatively well-maintained (which ends up meaning: relatively popular) so it would be updated for security fixes and also I won't have to "swap" distros often, have access to more repos (so I wouldn't be waiting "forever" to get the latest version of something, or have to install flatpaks much), and I wanted to avoid Ubuntu-based distros based on what happened with Unity in the past (that's me being overly picky, but it left a Microsoft-flavored taste in my mouth). AVlinux and KXstudio were recommended but it seemed they both didn't meet a couple of those goals of mine (I say "seem" because I'm only going by what I read on forums...I'm new to this, so no offense to those distros!)
I chose MX Linux. It also allowed me to try low-latency kernels from the repos (as well as other kernels), so I figured let's give it a try.
After doing some tests with Reaper's own plugins in Windows 7 and saving my results, I migrated to MX Linux (in October) and then did the same tests. My audio system of choice in Linux is ALSA (since I have no need for JACK).
My audio card was capable of lower latency than in Windows (with manufacturer-specific ASIO driver), using the stock MX Linux kernel, so that's a bonus I didn't expect. Also my CPU usage in these tests (apples-to-apples, Reaper using its own plugins) was more efficient by around 30%! I didn't pay attention so much to the CPU usage at first because I could just "load down Reaper" and see "when it started to break up". Even if my CPU usage meter in Windows wasn't close to 100%, I could get dropouts, so I knew it wasn't all about whether it showed 100% usage or not. In Linux not only could I push the CPU right to 90% with no problems at all (stable, smooth, no dropouts), I could see more even loading of the cores/threads (in Conky, etc.) compared to in Windows.
In Linux I did have to set my CPU frequency governor to "performance" (so it wouldn't "throttle down" to a lower speed), something akin to what I'd done in Windows' "Power options" using the "high performance" plan. But that was it, no further tweaks necessary. (I did have a graphics issue so I had to tell MX Linux to use the Intel-based driver for my onboard GPU, but that was something I figured might happen, and it didn't affect the performance in Reaper anyway.)
After some more testing I decided to swap kernels. First was Liquorix, for low latency use (or so I'm told). I did the necessary changes to ensure it would work for low latency (a couple changes to limits.conf). I tested after I did that, before swapping kernels, and I saw no performance difference (there might be, but it would be only at the very edge of my CPU when pushed to its limit). Then I installed Liquorix and rebooted. During my test in Reaper I noticed it was about as stable at high CPU use, but it didn't allow me any lower latency for my audio device and it actually increased my CPU usage noticeably (around 15%). I swapped back to the stock MX Linux kernel, and confirmed this. Later I swapped to an AntiX kernel (since it's updated with the latest Spectre/Meltdown mitigations and the stock MX Linux kernel currently isn't), and I did the same test. The AntiX kernel performed on par with the MX Linux kernel. Later I updated the Liquorix kernel and tested it again. I got the same results as before.
I've also tested all these kernels with Reaper using JACK and got virtually the same results.
My takeaway, as anecdotal as this is: I don't think I need a specific low-latency or realtime kernel. The one I did try only seemed to increase my CPU usage. I can't say why, but there it is.
Also: did you set your CPU frequency governor to "performance" (or at least check that it is)? If not, I'd recommend it.
I hope you can use this info somehow, maybe it points you in the right direction.
PS. a great reverb which is also very reasonable on CPU:
Dragonfly (try that latest/experimental version, since it allows for 10 second-long decays!)