Still new in Lightroom - slow display, slow download

Hi Community,

even I meanwhile in my "second paymentcycle" of Lightroom I'm still quite a newbee. I often have to issues, slowing me down and make working with LR a mess before I really can enjoy it's great features. 

1) Very often it needs a recognizable time to display a foto in full resoultion (usually around 15 MB shot with Canon 6D MII). It often tooks around half a second to shown fully. If you scan a bunch of 500 fotos first time, sorting out the crap, it's a definite showstopper if you have to wait each time half a second before you know if the foto is blurry or just not fully displayed. Also, when I went for and back usually I have againd to wait, so there seems no connection to a short-term-memory.

2) Uploading fotos can take it's time, but after I worked with them the download back to my PC is really a mess. Not only it take long time, it also takes all resources from my Computer. I cannot do anything else while back-loading. As usually loading from web-to-Compuer is the fast thing I don't understand why.

In both cases I assume the reason is that I upload the fotos to Lightroom, work with it online and then load it back. But I have no idea how I work with my fotos offline and maybe it's just necessary to upload, if the work is indeed not done by my PC but by some LR-Server. But I have no idea about the technic behind.

If someone have some good hints to solve the problem, I would be very glad, as it's really frustrating to be stopped out before you really can work.

Thanks in advance!

Detlef