Skype's video and voice-calling features become more of a hassle than convenience when suffering from lag. The culprit is likely either your Internet connection or computer's hardware. Ensuring your connection and computer adhere to Skype's suggested standard improves your chances of having an uninterrupted call. Lag also can be caused by other open programs on your computer.
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Skype's support site recommends a download and upload speed of 100 kbps each for smooth voice calling. The recommendation jumps to 300 kbps for video calling and screen sharing. For HD video, Skype suggests 1.5 mbps.
If your Internet connection is up to par, your computer's hardware still might struggle with Skype. Skype support suggests at least a 1 GHz processor, 256 MB of RAM and DirectX 9 or higher to optimally run Skype.
Skype's support site suggests closing open applications that use the Internet as well as stopping any downloads. Similarly, your computer's hardware may be under strain from other running programs, causing Skype to stutter and lag. Even if a program doesn't use the Internet, consider closing it to free up computer resources.
If nothing seems to help and lag only occurs with certain people, the problem might be on their end. If you're connected over Wi-Fi, consider getting closer to your modem or router, which may increase reception and speed.