Inkscape 0.92.1 is slow for copying or deleting lines with markers

[Kristof Mulier]

My system is as follows:
 - OS: Windows 10, 64-bit
 - Motherboard: Asus Sabertooth Z97 Mark1
 - CPU: Intel i7-4790K 4.00GHz, water-cooled
 - GPU: Nvidia Geforce GTX 960

I use Inkscape 0.92.1 r15371.
I have tweaked everything for speed in Edit > Preferences > Rendering:
 - Nr of threads: 8
 - Rendering cache: 4096 MiB
 - Gaussian blur quality: Lowest
 - Filter effects: Losest

I even hide most of the drawing using layers and removed the rulers (Ctrl + R). Despite all these efforts, inkscape gets slow when the file gets big (current file is 4MB). The whole drawing experience is slow, but two things are very very very slow:

 1. copying objects, especially lines having markers at the beginning or end.
 2. deleting objects, especially lines having markers ...
 3. Handling text.

Clearly, the markers on the lines make the difference in speed. Why? I have no idea.
My file has A LOT of lines with markers. That's because I'm drawing an electronics schematic, and the markers are dots to indicate connected signals. Seems like inkscape doesn't like lines with markers in big files... Also handling text gets slow.

In bug nr. #1665158, I read that File > cleanup document would solve the issue. But it doesn't help for me.

Please help

PS: I have added an example file with lots of markers. Please copy-pase a line without markers, and one with markers. You will notice the difference!