glfwSwapBuffers slow every 4th cycle

[Aisaaax]

Hello!
I've recently made another project on OpenGL-ES for ARM with LWJGL. And I am using GLFW now (I was previously using EGL rendering with no issue)

But I noticed noticeable stuttering in my program. I investigated it and found that every 4th cycle of my main loop - glfwSwapBuffers(window) takes A LOT longer than in other cycles. We are talking of the difference between 0.0003 seconds and 0.28 seconds.

I have no idea what caused this. As you can see from the code (below), I am explicitly testing the time it takes swapBuffers to work. I can guarantee that the rest of my program (which is extremely small BTW) doesn't really take that much time at all, and nothing should cause any issues. I think.
I'm using LWJGUI, which is also a first time for me, but I tested its performance and it seems like it's doing completely OK.

Vsync is off, but I think double-buffering is on.

Here's my loop:

while (running)
        {
            context.updateGlfwWindow();
            Vector2i windowSize = context.getFramebufferSize();
            glClearColor(1, 1, 1, 1);
            glViewport(0, 0, windowSize.x, windowSize.y);
            glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

            renderer.render(frame, context);

            glfwPollEvents();

            double timeStart  = System.nanoTime() / 1000_000_000.0;

            glfwSwapBuffers(window);

            double timeEnd = System.nanoTime() / 1000_000_000.0;
            double delta = timeEnd - timeStart;
            System.err.printf("%f\n", delta);
            
            initializer.getSystemEventProcessor().processEvents(frame, context);

            initializer.getGuiEventProcessor().processEvents();
        }

Here's the sample output (timings). notice that the dip happens exactly every 4th time.

0.000372
0.000378
0.000370
0.274361
0.000344
0.000367
0.000395
0.275922
0.000393
0.000365
0.000370
0.274104
0.0003989
0.000477
0.000393
0.273459

[Aisaaax]

update:
I tried limiting the loop, by adding some basic FPS control there. Basically I check the time at the end of the loop, and if it's not yet the time to have another frame - I simply wait. My idea was that perhaps the videodriver gets a bit overwhelmed by all the calls and chugs periodically.

Unfortunately, this didn't solve the issue. But now the stutter happens every 8 cycles instead of 4.
So the slower the loop runs - the less frequent those stutters are. But they are still simply inexcusable - 0.3 seconds is as you understand - way too much. It's like 3FPS which is 10 times worse than what I'm aiming at.

Also I want to stress that rendering seems completely fine otherwise.

[Aisaaax]

Narrowed the problem down to LWJGUI/ImageView. For some reason, I can render lots of simple elements, like buttons, but if I render images it just hickups every X'th frame.
I have a suspicion that perhaps something like NanoVG or something else causes images (textures) to re-load periodically. But it is interesting that glSwapBuffers gets affected.