There have been a few people on here thinking that Sketchup Free needs a really fast connection. Admittedly you will need a reasonable speed and it will have to be available whenever you want to use Free. The delays I see are not waiting on anything downloading. They are waiting on the program running. It uses quite a lot of CPU time for me, especially when starting up. I have quite a fast internet connection, just tested at 165Mbps down, 10Mbps up but find it hard to see much activity there when Sketchup Free is starting up. I think it peaked at 8Mbps once, but most times its hardly downloading anything at all. It is possible that it is waiting on downloading if Sketchups servers are struggling.
My CPU usage is noticable though. I get 1285% (using 13 virtual cores out of 16) for about 5 seconds, then settles down to just over 100% for the rest of the start up time. Even once I’m able to start modelling, it stays at 100% for a while, then settles down to almost nothing, like 5% even while modelling.
My GTX 970 graphics card isn’t working hard either, barely hitting 20% processor usage and about the same for its RAM.
Performance isn’t too bad at all for me. @mjeanson said it struggled with a model with 10,000 polygons so I tried one of my files which has over 1,000,000 edges and 525,000 faces. It is pretty slow to load, but once I’d got zoomed in to the model, it was quite usable, especially turning off the layer with the most polygons in it. It is much slower than Sketchup Pro on my Mac and also slower than the Sketchup viewer on my iPhone 6 which is a bit sluggish with this file but not as bad as Sketchup Free.
I’m using Safari 11.0.1, Mac OS X 10.11.6 on an 8 year old Mac Pro with 2 x 2.26GHz Quad-Core Xeons, 48GB RAM, GTX 970 4GB.