When you open three or more editors at once third one and every further will be very slow, but first two opened editor will perform as usual. If you close first two editors third one will be still slow you will need to reopen it.

Also, if you close first two editors (1 and 2), but third will remain opened (3) it will be still slow, but if you open one more (4) it will be fast again. If you open even one more (5) it will be slow as well as (3).

It looks like this issue is reproducible only on Google Chrome.