Hello @alex_khan, I think I’m answering your query first time on this forum. So let me brief you a little about me. Umm. Oh no you can directly check it out on my profile.
Coming on to your query, what I want to tell you is that there are no pros and cons of using Light box in blogger on your blog’s SEO.
It don’t improve it but also don’t affect it.
So you need not worry about the SEO part.
Then coming on to the page speed. Well it can increase your page size by 0.2 to 0.5 kb but not more than that. So you need not worry about that also.
And advantages of using it to improve user experience has been already told to you. Thanks to my other blogger brother for it.
Stay in touch.