Subtitle upscaling causes frame drops

Important Information

• mpv version

mpv 0.35.0-339-gd1d0b1a8 Copyright © 2000-2023 mpv/MPlayer/mplayer2 projects
 built on Tue Mar 28 12:41:04 2023
libplacebo version: v5.264.0-85-g0f36b01
FFmpeg version: N-110105-g9bf1848ac
FFmpeg library versions:
   libavutil       58.5.100
   libavcodec      60.7.100
   libavformat     60.4.100
   libswscale      7.2.100
   libavfilter     9.5.100
   libswresample   4.11.100

• Windows Version

Edition	Windows 10 Pro
Version	22H2
Installed on	‎19/‎03/‎2023
OS build	19045.2728
Experience	Windows Feature Experience Pack 120.2212.4190.0

• Source of the mpv binary
  https://github.com/shinchiro/mpv-winbuild-cmake/releases
• If known which version of mpv introduced the problem: No
• Possible screenshot or video of visual glitches: No

Reproduction steps

mpv --no-config --fs --log-file=Example1-0005.txt --gpu-debug --vo=gpu-next --gpu-api=vulkan Example1-0005.m2ts
Use mpv to play 1080p video file with subtitles on a 4k display using gpu-next and gpu-api=vulkan.
This issue seems to not exist if playing on a 1080p display, which makes me think that subtitle upscaling is the trigger.
Example 1: Issue starts from 00:05 (5 seconds in the video).
Example 2: Issue starts from 03:32.
Example 3: Issue starts from 16:33, this case is the worst, no subtitles visible on the screen and so many frame drops.

Expected behavior

No frame drops.

Actual behavior

Frame drops.

Log file

Example1-0005.txt
Example2-0332.txt
Example3-1633.txt

Sample files

Subtitle file:
Example1-0005.zip
Example2-0332.zip
Example3-1633.zip

Reproduce the issue with the provided mpv command using gpu-next, Vulkan, a 4K display, and the linked sample files. Start by comparing Example1-0005.txt, Example2-0332.txt, and Example3-1633.txt around their reported timestamps. Done means identifying and addressing the subtitle-upscaling condition so playback has no frame drops.