I am running an HP Micro Server for over an year now, with owncloud/nextcloud. The server is mostly an backup solution with nextcloud as an aditional service that is separate from my backup software. Performance wise there are much more factors to consider than just the bare metal server… Do you plan to use nextcloud mostly in your home or remotly? For home usage, your connected devices only should use 1GB connection. If you still have some 100MB connections than for example your network can be a limiting factor. For the WebInterface perfomance: A lot comes to caching. I am running a redis server on the G8 and it can help loading big chunks of image previw much faster. But only if it does it in advance.
The best performance is if you catched all images for example in memory, but you need than a lot of memory or some sort of inteligence, that know what to cach before you look at it. Than your experience with the G8 is stunning. For viewing larger directorys of images i am using the webinterface more than the app, i dont like it that the app always trys to download all the files before viewing to the machine ( On Tablet or Smartphone) . I think there is not much of a bandwith difference in directly downloading the images or using it via web. But the way the App handles it is not as nice as an full cached webinterface with image preview. The Webinterface is much more suited for that kind of job. So if you realy want to use the Webinterface for browsing, you need to learn how much a bottleneck php for example is on your system. how to tweek the caching, fitting to your needs and so on. The G8 is only one piece in the puzzle. I never looked back to buy it, because it is realy great bang for the bug, but consider other factors as - caching RAM, your network situation (internal and external), different php version, image caching and so on. There are many task to solve. You can have an working enviroment with all the installation defaults. The Webinterface will be usable but probably not as a sharp tool as it could be.
Are you a Nextcloud newbie or overall an Linux(admin) newbie?
For the most part you will not need that big insight view, because the guys here did a greate job making the experience as simple as it could get, to run your own server. But you realy should try to understand every step in the Nextcloud Administration Manuel https://docs.nextcloud.com/server/12/admin_manual/. Go to the tuning section and ask yourself why they do certain things, google your questions that will help you to get a better picture. In the end nextcloud is just a software that is running on top of a lot of other things.
tl:tr you get a default usable webinterface on the G8 but if you want to get to town you need to be willing to spend time resachring and digin in your enviroment for bottlenecks.
Hope i could help a bit, i am only an advanced noob myself