There are a few reasons why Snapchat videos lag for you, and I'll give a brief explanation of them below.
One of the major reasons for Snapchat video lag is a connection issue. If your phone is struggling to find a strong connection, you simply can't play videos without the chance of lag occurring.
To see if this is the case, you should try to play videos on different apps to check whether they're having the same issue. If not, it could be a problem with the Snapchat app itself.
Snapchat app problems
If you're connection isn't the issue, then it could be a problem with the Snapchat app that's causing your videos to lag. Sometimes apps decide to be irritating and not work properly.
A simple method to try to rectify this is to completely delete the app and then reinstall it. If you're Snapchat videos are still lagging after that, it's probably not the app at fault.
Is your phone at fault?
The final thing I would suggest trying is to reset your phone. I'm not a technical genius by any stretch of the imagination, but I find that a phone reset often fixes my issues.
Make sure you back up any files you need before you reset the phone, however, as you won't be able to get them back after the reset.