After much amateurish research, I’ve come to a conclusion - the only way for me to not get laggy playback using H265 content (variying resolutions, though with HDR 10) through Infuse is by disabling subtitles. This is through an ethernet connected aTV4K 2017 with average speeds of 450Mbps, connected to a small Win 11 server via SMB. Infuse 7 on the latest available update.
Is there anything related to how Infuse draws subtitles on HDR playback that has changed in the last few versions? I can’t remember this being a problem about a year ago.