If the problem were on eBay's end, would you not expect others to experience the same issue?
Can you explain what "lag" you are experiencing, exactly? Which things seem to be slow?
For a while my eBay account Site Preferences page was very slow to load -- typically taking sixty seconds to appear -- but other than that I have not noticed any particularly slow loading pages recently.
Any difference if you use another browser?
Many things might cause an individual site to load slowly -- cache issues, browser settings issues, cookie issues, add-ons or extension conflicts. An ad blocking program with an overly broad custom filter might cause problems, for instance. An antivirus program that has misidentified a site feature as a malicious program might cause a problem. A network blockage somewhere between you and eBay could slow things down but might not necessarily affect other sites.
If the problem occurs on a second device or computer, that would rule out many issues, but not necessarily those that involved common factors that are the same on both devices, or common network equipment. An ethernet switch with a bad routing table could cause all sorts of problems; rebooting common network devices might shake things loose.
In addition, there are things like adware or malware that can cause problems. Some adware is triggered by going to certain popular sites like eBay or Yahoo, and then various ads attempt to load. If the ad does not load properly, that can slow things way down. A scan of your computer with something like adwcleaner by malwarebytes might be worth a try.
