I'm not the parent commenter, but I work using Figma (and other design tools). So, I may be able to clarify it.
Figma is a vector drawing tool like Illustrator, which means that the comparison with Photoshop layers is misleading. What the parent post refers to as "Layers" is the name that Figma uses for objects like rectangles or text. So if you have a design file with many elements the number of layers grows quickly. The same happens in Illustrator or Sketch.
The closest equivalent to merging a layer is combining different shapes in one or rasterizing vectors into pixels. The usual tricks to handle big files are to work with different pages (so the app doesn't have to render everything at once) or to rasterize the parts that you'll like to keep as a reference (and move the vectors to another file).
It's not a problem of Figma, but a feature of vector design tools where you like to preserve the editing (and details) of everything. You'll have the same performance challenges in Sketch, XD, Illustrator, Affinity Designer, etc. The case of Penpot using SVG is interesting because big files will push the Browser SVG to its limits. (BTW I took a look at the Penpont code, it's interesting that they use ClojureScript for everything... it gave me the motivation to learn about CloujreScript).
Figma is a vector drawing tool like Illustrator, which means that the comparison with Photoshop layers is misleading. What the parent post refers to as "Layers" is the name that Figma uses for objects like rectangles or text. So if you have a design file with many elements the number of layers grows quickly. The same happens in Illustrator or Sketch.
The closest equivalent to merging a layer is combining different shapes in one or rasterizing vectors into pixels. The usual tricks to handle big files are to work with different pages (so the app doesn't have to render everything at once) or to rasterize the parts that you'll like to keep as a reference (and move the vectors to another file).
It's not a problem of Figma, but a feature of vector design tools where you like to preserve the editing (and details) of everything. You'll have the same performance challenges in Sketch, XD, Illustrator, Affinity Designer, etc. The case of Penpot using SVG is interesting because big files will push the Browser SVG to its limits. (BTW I took a look at the Penpont code, it's interesting that they use ClojureScript for everything... it gave me the motivation to learn about CloujreScript).