Welding or unwelding a part (even if it is massless) to the limb of a humanoid that is not in its default position (default position being the position it would be in without animations) causes a speed glitch 100% of the time. The further the limb is from its default position, the greater the effect is. I expect that massless parts should not affect the character but it does. This works with anything that gets welded, including tools, meaning that in any game with a tool that has a handle you can get a speed glitch (some games have fixes for it, however).
This happens in both studio and live games and with both R6 and R15.
I am unsure how long this glitch has existed, but definitely for a long time. I remember this glitch was prevalent years ago in certain games.
Simple repro using a tool
- Join a game with a massless tool (there is a provided place file below).
- Play any animation that results in your right arm moving (the more, the better). I recommend the R6 dance2 emote. When your arm is offset, equip the tool.
- Jump around with shiftlock or first person, your movement will be affected and you can go faster when holding certain directions, depending on the animation.
SpeedGlitchReproFile.rbxl (23.4 KB)
Here is a video showing what it looks like. I can do a 13 stud jump, something normally very difficult as you can see at the start, with ease when using the speed glitch:
Tip for seeing if you have a speed glitch
It can be hard to tell if you have a speed glitch sometimes, especially when it is small, however, there is an easy way to tell. If you jump backward with a speed glitch, you will start to move sideways randomly. Thus, if you perfectly align your camera to 90 degrees (the place file has a script that lets you do this using the , and . keys) then you can jump backward, and if you move sideways at all then you know you have a speed glitch. Here is a video showing this:
Workaround
There are ways to fix this with a script, but they are hacky. The best way I have found is that disabling then enabling the root joint creates the same effect as welding a part to the humanoid, thus if you force the joints to return to their default position for a frame before doing this you can remove any speed glitches. Here is a model where I do this upon tool equip, note that this model only works for tools.