VLC 3.0.14 Windows 10 b19042 - very slow startup playing video files, hangs for X seconds when seeking or toggling fullscreen

If I open a video file (typically Matroshka x264 or .MP4 ditto, or MPEG-4/XVID AVI), I get the VLC window showing a black frame and with the time counter stuck at 0:00:00 - for approximately 4-5 seconds before the video starts playing. If I then attempt to seek to a later/earlier point in the video (by clicking on the timeline), the video - about 50% of the time - freezes for a few seconds again, although the audio usually continues. If I attempt to go fullscreen, I get the playback controls/bar at the bottom of the screen, but still see the Windows desktop with the VLC window at normal size within it (i.e, VLC thinks it's fullscreen, but isn't rendering the video at all). I have to toggle fullscreen/windowed a couple of times before VLC will begin playing video again.

This is with VLC 3.0.14 Win64 on Windows 20H2 b19042 x64, fully patched as of 5/27. Hardware is Intel Core i7 9850H/Intel NUC BIOS version QNCFLX70.0061, using an ATI Radeon RX560 driver version 20.9.1, video & audio are output over HDMI. VLC is running in DirectX11 video output mode.

This problem manifested immediately when I upgraded from VLC 3.0.12 to 3.0.14.

I have downgraded back to 3.0.12 and can confirm that the problem isn't present on the older version - playback begins and seeks instantly, and I can flick back and forth between fullscreen/windowed without issues.

I was prompted to create this issue after reading this post on the VLC forums from other users who are seeing similar behavior but didn't wish to create an issue.