Not the parent commenter, but I also tried it a year or two ago and my recollection is that I had a similar experience. I have a really long history file that dates back like a decade and if remembering correctly I tried to import it, and it made using my shell very laggy so I ended up removing it. I'm not 100% positive that I'm remembering the exact way that I started experiencing the lag though, so if I have time tomorrow I'll try it out again and see if it's still something I encounter.