Just adding to the discussion and maybe useful for tracing down the cause: I have the exact same issue on Windows 10 using the Pianoteq VST in Ardour, soundcard Focusrite Scarlett 2i4.

That means you are experiencing issues with VSTi or VST3 versions. The OP is having issues with the LV2i version in Linux.

In both cases I would make sure that the DAW hasn't inadvertently hiked up the sample rate - I tried a software synth yesterday and running that the sound changed to 192khz instead of 48khz. Once that happens I cannot run as many tracks and issues like the OP described would rear their ugly head. Actually it was struggling with a single instance of the synth until I dropped it down to 48khz (I didn't try anything between). 
OTOH if I only run a few instances of Pianoteq and nothing else I could probably get away with the ridiculous/pointless 384khz sample rate, add other more hungry VSTi tracks in the mix and the system would be crippled.

Indeed your Scarlett shouldn't have those issues.
It helps to tell people what digital audio workstation you are using and what kind of sound interface upfront. Saves needless speculation.

Other than increasing the buffer size (accepting slightly more latency -needn't be noticeable) and freezing tracks in your DAW then consider sharing FX across tracks on auxiliary bus rather than separate instances per track.

It could be one of your VSTs is very CPU hungry. Process of elimination there..
Hopefully someone that knows Linux will help you.

Possibly related conversation..
https://linuxmusicians.com/viewtopic.php?t=19322

Good to hear that it seems more efficient for you. ☺️
On the other hand it doesn't really sound like it's being terribly efficient for you right now!
Your i5 should handle multiple instances of Pianoteq 7. Of course we all know that depends on just how many other VSTs you've got running at same time.

Something else using the sound device at same time?
Have you set the sound device to exclusive mode or shared? - the latter can cause problems.

Sorry if this isn't all immediately helpful, just some comments.
I'm interested in seeing where this goes, I'm not a Linux user yet but considering it.

Perhaps your sound interface only likes to work at a given rate? Some only work fixed at 48khz.

Is there an a version of ASIO for Wine you could try?

That's an interesting hardware combination. Massive memory for a relatively modest CPU - You might have unusual requirements?
Given memory prices have been stubbornly high these last few years it makes me wonder why such a combination?
Might make sense if you use an enormous amount of low CPU sample instruments or you work on very large 4K video projects or massive CAD work - but then you would be in need of an i7 to cope with that too.
I would have built with a more powerful same generation i7 and 16-32GB to get the best CPU you can afford - using larger memory sticks to leave space for expansion.  16GB which is overkill most of the time is a good margin to have. In short it's not a good idea to skimp on the CPU to afford even more memory for most tasks.
 
However that shouldn't be the problem here.  Your CPU is obviously capable of running multiple instances of Pianoteq and several other VSTs simultaneously.
So I suggest going to 48khz, exclusive modes for sound and testing that if you were using something unnecessarily high for your projects like 96khz-192khz?

Thank you for the reply, Key Fumbler.
The memory was recuperated from a previous computer. I thought I'd need it for my large sample libraries, but now Pianoteq and Swam Strings changes all of this.
Changing the sample rate through the DAW does not change anything. My sound interface is a Scarlett 2i2. There are no drivers in Linux to specify the audio interface's sample rate. There is no ASIO either, but latency in Linux is as good without it. So I'll go in Linux's configuration file to change the sample rate.
By the way, CPU performance in my DAW is SO MUCH better since I adopted Linux. Pianoteq and all plugins are less CPU hungry, with graphs as evidence!
I'm not too concerned about the Wine version I'm using (6.40; updated versions are rollbacks) because everything (except the SWAM plugins) run natively.

Hi, got Pianoteq about a week ago. Fantastic instrument.
Here's my problem. I'm on Linux Manjaro. My DAW is Reaper.
My system is a core i5-8400, 64GB memory.
Everytime I load a project with a Pianoteq instance, the instrument's volume is dampened and sounds like there's a bitcrusher on it (sound like at 22KHz).
When I hit the panic button (!), all is good. But I have to do that everytime I load a new instance of Pianoteq.
I use the LV2i version of Pianoteq, not the VSTi.
Any advice?

Something else using the sound device at same time?
Have you set the sound device to exclusive mode or shared? - the latter can cause problems.

Sorry if this isn't all immediately helpful, just some comments.
I'm interested in seeing where this goes, I'm not a Linux user yet but considering it.

Perhaps your sound interface only likes to work at a given rate? Some only work fixed at 48khz.

Is there an a version of ASIO for Wine you could try?

That's an interesting hardware combination. Massive memory for a relatively modest CPU - You might have unusual requirements?
Given memory prices have been stubbornly high these last few years it makes me wonder why such a combination?
Might make sense if you use an enormous amount of low CPU sample instruments or you work on very large 4K video projects or massive CAD work - but then you would be in need of an i7 to cope with that too.
I would have built with a more powerful same generation i7 and 16-32GB to get the best CPU you can afford - using larger memory sticks to leave space for expansion.  16GB which is overkill most of the time is a good margin to have. In short it's not a good idea to skimp on the CPU to afford even more memory for most tasks.

However that shouldn't be the problem here.  Your CPU is obviously capable of running multiple instances of Pianoteq and several other VSTs simultaneously.
So I suggest going to 48khz, exclusive modes for sound and testing that if you were using something unnecessarily high for your projects like 96khz-192khz?