Media Encoder 2021 15.4 Build 42 huge strain on CPU, slow rendering

Don't know if nayone else has had similar issues but the latest Media Encoder  is slower and more resource hungry than previous versions.

I regularly encode a project from 1080p/50 PoRes 422 files. Up until recently 47 mins of video (split in 4 separate files) would take arounf 45 to 55 mins to encode to MPEG2 576 25i files. Hovever in the last few weeks this has incresed to between 85 and 116 mins. During the encode the fans are on full blast and mouse movements are slow and semi unresponsive,

As an expriment and as I had not uninstalled Media Encoder 2020 I ran the same project, this time it took 39 minutes with no excessive fan noise although they were certainly still chugging away.

I am using a MacBook Pro 16" 8 Core i9, 16 GB RAM with AMD Radeon Pro 5500M 4GB, There is also around 550GB of free space on the SSD system drive but the encoded files are comng from a 6 month old  connected 7200RPM GTECH desktop HDD connected via USB3.

All apps closed except Media Encoder and in the background Norton 360, Adobe CS and Microsoft One Drive are running. Currently running OSX 10.15.7 and not Big Sur so maybe this is an issue?

So work around is use the old version of Media Encoder but any ideas?  Also as a comparrison no speed issues with Apples Compressor although not like for like files.