Lagging and Crashing on short documents (I experience this issue only on two documents and I cant pin down why)

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

On chrome and also on explorer I have 2 markdowns which lag, one of them is very short under a page, the only thing distinct about them might be the variety of symbols used. I have other markdowns longer and also with plenty of math env that do not lag.

Eventually the page crashes (get the promt of wait or exit page from chrome) and does not move at all.

#### Current behaviour

Provide steps to reproduce:

1. Go to https://hackmd.io/n1dGhDgNQr6Q_SOjDPw4Cw or https://hackmd.io/6Ox_r9xkQoGh1LSFi618xQ
2. Scroll up and down the document several times
3. Eventually the docuemnt starts to lag until it crashes
4. I could just use the pen mode as other issues have suggested, but its a short document part of the advantage of hackmd was the double view.
5. If you look at this hackmd https://hackmd.io/wiHD0O4uSDCGmCajE9l1Iw it does not have the same problem, I have longer ones two that are similar up to 3 pages maybe and again no issue. I cant quite pin down why the two linked above are lagging.

#### Expected behaviour

As I mentioned above I would expec things to behave like the md in 5. , no lags no crashing. I cant quite pin down what the differences between the mds in 1. and 5. are, at some point I felt it was due to me adding a specific macro at the start (I have most these macros / defs in all my mds and its not causing a lag).

#### Environment

I use HackMD on:

**Dell XPS Laptop**
- OS: [eg. Windows 10 Pro]
- Browser: Chrome
- Browser Version: 96.0.4664.110 (Official Build) (64-bit)

Has happened on other machines with Linux enviroments too also on explorer.