I think the general advice is still "multiple manually created versions are better than onthefly transcodes"
transcoding on the fly has massively improved; especially with an Intel GPU (for that cheap, unlimited AV1)
but if your sources are 4K HEVC and HDR; clients are probably asking the server to transcode to 1080p, H264 and then tonemap the HDR to SDR
not sure what'd cause Plex to have better transcode performance, maybe the client just allows higher bitrates by default over the internet