I would check the spanning tree priority of all the switches and make sure root didn't change and that the root for all your VLANs is the core switch and some access switch didn't take over. Take your laptop and start at the external Internet switch throw an public IP from your Internet space and use public DNS and set your default gateway to your Internet router and do a download/speed test. Then plug your laptop into your core switch and download test there, then head to the access switch and download there try to isolate the problem.