Opening more sequencer timeline areas will cause bad playback rate, but in preview mode it doesn't.
Have a Video Editor area open with a Sequencer and Preview and notice the playback rate.
Open another Sequencer area and notice the drop in playback rate.
(Only Sequencer areas will have this effect, but not Sequencer Preview)
As seen in this gif:
Operating system: Windows-10-10.0.17763 64 Bits
Broken: version: 2.81 (sub 11), branch: master, commit date: 2019-09-14 14:25, hash: rB38cc53a168d2