Notebook crashes or slows down

### Steps to reproduce
1. I dont know
### Expected behaviour

The notebook should run smoothly after many hours of measurement
### Actual behaviour

The notebook hangs, or even the whole chrome tab crashes. Maybe also what happened in QCoDeS/Qcodes#175
For the tab crash I had a hard time to reopen the notebook, as this java widget, or any cell or whatever had a lot of `negative delay` messages.

When the notebook finally loaded after many trials and waits it was super slow, scrolling was a pain, I could resolve this by restarting the kernel and clear output.

I also see my notebook slowing down when reusing it for longer times, so I end up making copies or from now on clearing outputs.
### System

**operating system**
Win 7
**qcodes branch**
Merged Master with merlins instruments, array_ids

NO plotting enabled

No file or test is named. Start by reviewing the long-running notebook behavior described here and QCoDeS/Qcodes#175, then reproduce repeated measurement use with plotting disabled while observing kernel, output growth, and browser-tab stability. Done means the notebook remains usable after many hours without hanging, crashing the tab, or requiring output clearing and kernel restarts.

Written by the indexing model from the issue text.