What is Firefox request to ocsp.int-x3.letsencrypt.org and similar

When I test speed in Firefox, there are some strange requests tied with SSL certificate at the beginning of the waterfall:

[]ocsp.int-x3.letsencrypt.org when cert issuer is Let’s encrypt
]ocsp.globalsign.com/rootr2 and ocsp.comodoca.com when issuer is Globalsign (nginx.com) https://www.webpagetest.org/result/180116_TK_77e6763b66a6e719f94ff74bc7baf78f/

But it isn’t always present in waterfall. For example site troyhunt.com (Comodo is cert issuer) has no such request: https://www.webpagetest.org/result/180116_KW_74a216e45acd585bbae822967ec43e5e/1/details/#waterfall_view_step1

May somebody explain me what it is? Can I somehow reduce these requests? Why chrome doesn’t do such requests?

It’s the check as to whether a certificate has been revoked or not - Online Certificate Status Protocol or OCSP for short

Different browsers have different rules as to if and when they check them.

A technique called OCSP Stapling will remove them, which needs to be configured for the root domain on your server, third-parties need to be configured on their own servers / CDNs

That it was, so I have enabled OCSP Stapling on all my servers, thank you.