I have the problem as well. Apparently no one has been smart enough at Samsung to fix it considering its been going on for a long time. Should of stuck with my other phone.
Try updating your messenger app. If the app is updated, please try the steps below.
Still nothing. App is up to date. cleared cache cleared date. GIFs still play fast. Whats odd is I sent a couple of them and some play reg speed but most played fast. Tried letting it play before I pressed send, sending it as soon as it let me, sent it to Iphone users and sent it to Galaxy users, all have the same issue.