Rendering becomes slower without pane frames

[raphCode]

Something I discovered while stress testing, probably not a relevant use case but here we go:

• Start new session
• open a lot of panes, for example by holding / spamming Alt-N until no more panes can be opened
• Observe as the typing performance degrades when the pane frames are disabled: Ctrl-P, Z

One can also enter Sync mode: Ctrl-T, S and enter a command that creates a lot of differing output, like curl parrot.live to see the performance drop even without typing.

[raphCode]

This issue helps to reproduce #1813 and should be fixed after the mentioned one.