Interesting that I still find posts today (October 2017) where people complain about this issue. I'm one of them. I'm running El Crapitan 10.11.6 with Safari 11.0. I had this lag on Yosemite and El Crapitan as well. Turning off iCloud, etc did not solve it for me. What it was/is the AdBlock Pro extension. That's the only extension I have in Safari but seems to somehw interfering with opening new tabs. I also have my favorites listed in the dropdown list when you click in the address bar. That was slow as well. Took 2-3 seconds to have them drop down. All this issue has disappeared when I disabled AdBlock Plus. Will have to figure out a strategy to keep ABP but so that it will not interfere with tabs and favorite dropdown.