I am having the same problem described in a post by @TerryHibbert on September 29, 2017 here: https://forum.webflow.com/t/designer-slow-while-editing-symbols/48129
When I double-click a symbol to open it for editing, the Designer starts lagging badly, taking about 5-7 seconds to respond to each mouse click. When I close the symbol the Designer works fine.
I found the problem goes away when I log into Webflow using Incognito mode with browser extensions turned off (except the Webflow extension), as suggested by @Brando in a response to the other post. Is there any way I can get rid of this problem other than using Incognito mode?
(Sorry if my etiquette is incorrect but the drop down that says “Please read the following before posting to the Bugs category” did not lead me to anything.)