Drives are all 7200rpm, the external connected via FW 800.
I created a new project and copied over bins to Mac HD, that seems to have really helped. I think I was breaking the cardinal rule about keeping project files and media on separate drives. Did not seem to matter much until footage grew.
No expert, just a guess.
And user settings made no difference.
Don’t think memory allocation came along until 8.5 as you stated.
Just given option to clear bin memory here.
Thanks very much to all!