I don't even know if this is being caused by this shader or a mod, but I'm grasping at straws.
So while playing I can run at 60-80fps normally for a while, but after a bit my fps will suddenly crash down to 3-5 fps, my console is then filled with these warnings and errors till it hits the 10000 line cap:

[14:37:30] [Render thread/WARN] [minecraft/Profiler]: Something's taking too long! 'root.gameRenderer.level.shadow terrain.render_cutout_mipped:RenderType[CompositeState[[texture[Optional[minecraft:textures/atlas/blocks.png](blur=false, mipmap=true)], no_transparency, diffuse_lighting[false], shade_model[smooth], alpha[0.1], depth_test[<=], cull[true], lightmap[true], overlay[false], fog, no_layering, main_target, default_texturing, write_mask_state[writeColor=true, writeDepth=true], line_width[1.0]], outlineProperty=affects_outline], ]' took aprox 172.4739 ms
[14:37:30] [Render thread/WARN] [minecraft/Profiler]: Something's taking too long! 'root.gameRenderer.level.shadow terrain' took aprox 172.8625 ms
[14:37:30] [Render thread/WARN] [minecraft/Profiler]: Something's taking too long! 'root.gameRenderer.level.terrain.render_cutout_mipped:RenderType[CompositeState[[texture[Optional[minecraft:textures/atlas/blocks.png](blur=false, mipmap=true)], no_transparency, diffuse_lighting[false], shade_model[smooth], alpha[0.1], depth_test[<=], cull[true], lightmap[true], overlay[false], fog, no_layering, main_target, default_texturing, write_mask_state[writeColor=true, writeDepth=true], line_width[1.0]], outlineProperty=affects_outline], ]' took aprox 126.8821 ms
[14:37:30] [Render thread/WARN] [minecraft/Profiler]: Something's taking too long! 'root.gameRenderer.level.terrain' took aprox 127.3093 ms
[14:37:30] [Render thread/WARN] [minecraft/Profiler]: Something's taking too long! 'root.gameRenderer.level' took aprox 304.0505 ms
[14:37:30] [Render thread/WARN] [minecraft/Profiler]: Something's taking too long! 'root.gameRenderer' took aprox 316.2293 ms
[14:37:30] [Render thread/ERROR] [minecraft/Profiler]: Profiler tick ended before path was fully popped (remainder: 'root'). Mismatched push/pop?

The shift+f3 looks like this:

Any help at all will be appreciated, even just a confirmation that it's not this shader causing it.
Also: Reloading the chunks resets the issue for a little while, and occasionally when I do so I get a GL error 1281 (invalid value)

My first advice is to update to the latest version (5.03), it has important changes in the handling of buffers, which may be related to the problem.

Use the latest version of your video drivers.

You can test if it is a mod by starting the normal game in a test world without mods.

At the moment I cannot reproduce the error.