This is generally dependent on the user's system, so the only surefire way to consistently improve it is to upgrade your system. That said, there will always be a point where the page is too big for the MathJax update to work smoothly.
This has been answered in the Mathematics Meta site, Feature request: user script or option to turn off instant MathJax rendering. Quoting Jack Schmitt's answer there directly:
I don't know how you want to do user scripts. I use bookmarklets
To pause it:
To turn it back on:
Presumably something similar can be done with automatic solutions like
greasemonkey. The turn-it-back-on does not request an immediate
render, but you can add the:
MathJax.Hub.Queue(["Typeset", MathJax.Hub, "wmd-preview"])
to the bookmarklet yourself if you'd like. Turning it back on may
cause a delay; if so I can adjust the script to more drastically
If you want to keep the updating but just make the wait longer for each refresh of the preview, this userscript should be able to do the job with a suitable delay.
If you are writing a very long answer, I find StackEdit to be a good tool, and it enables you to draft and save long posts in Markdown + MathJax.
As a more extreme solution, you can get Privacy Badger (which is a good thing to have anyway) and temporarily disable all content from cdn.mathjax.org to disable all math rendering.
If the lags persist even with MathJax disabled then that is definitely something to report.