Occasionally while sketching in SolidWorks you may not wish your sketch entities to automatically snap to existing geometry. This can be troublesome when sketching in a large assembly as there may be lots of other geometry nearby that the SolidWorks sketcher is eager to snap to. There are a couple of options in SolidWorks, enabled by default, that you could try disabling to see if it may help with this.
The options are in the SolidWorks system options under Tools -> Options -> System Options -> Sketch -> Relations/Snaps. They are titled “Automatic Relations” and “Snap to Model Geometry”.
With Automatic Relations disabled, then no relations are added at all (as the name implies). This can be helpful if you have a lot of sketching to do and don’t want any relations by default.
The second option “Snap to model Geometry” is interesting too. This one can be used for the case where perhaps you want relations created automatically to sketch entities only, not model geometry. Note in this case the relations will only be created to sketch entities within the same sketch.
Also consider customizing the snap settings in this dialog as well. For example, if you find that you never want to have a Tangent relation added automatically, disable it here.
Another related option is the “No External References” option on the Sketch toolbar when editing a component/sketch in the context of an assembly. When this option is enabled, you cannot make references between parts in an assembly.
One other bonus tip: automatic sketch relations can be temporarily disabled by pressing and holding the CTRL key while sketching. Once you release the key, automatic relations will occur again.