Not really, but luckily most discs don't. But I have observed it on a bunch. There are also weird cases sometimes, eg. on the Avatar BD, the normal subtitles are small section, but the forced alien subtitles are covering the full frame.
The other optimization coming will help quite a bit in either case though if you are playing on 4K, because PGS subs are always 1080p, even on UHD BDs, as far as I know, so if we don't upscale them anymore before the video renderer, its quarter of the size to handle and send to the GPU.
While I'm in there, I noticed we don't treat UHD BD subs as HDR10 like it should be, so thats something I need to address somehow.