This is a known issue on several devices. According to TikTok support, it's a bug in Snapchat. Some users have reported that uninstalling and reinstalling TikTok, then clearing the app cache and trying again fixes the problem, but it could be just on some devices like the Huawei P20, and the problem may resurface after a couple of shares. There's another workaround where the problem goes away if you download the video to your phone and then upload it as a new Snap. For now, this seems to be the best way. Or, you can also directly speed up video on snapchat.