'shift z' causes unsteady playback

Hello
I have imported a 90min doc that I edited in 12.5.
When I ‘shift z’ to see the whole timeline during playback, playback falls apart.
From a steady 25fps it drops to 8, 16 7, 10 etc and never makes it back to 25 unless I zoom in again.
Just one zoom increment is fine and playback returns to 25fps.

paul

macOS Sierra 10.12.6 (16G1212)

MacPro (Early 2009)
upgraded to…

Model identifier 5,1

Processor 2 x 2.46 Ghz 6 CoreIntel Xeon

Memory 32 GB 1066 MHz DDR3

Graphics NVIDIA GeForce GTX 1080Ti 11Gb

Mini Monitor 4K

Resolve 15.0.oB.065

[Peter Chamberlain]

What codec are the source clips?
What is the source and timeline res?

Hello
I have imported a 90min doc that I edited in 12.5.
When I ‘shift z’ to see the whole timeline during playback, playback falls apart.

Turn off the thumbnails in the timeline. Resolve attempts to generate thumbnails as the playback occurs which bogs the system down.