Hi - a little help here

Ok so, I recently got myself an iMac computer - the one I got is 3.8ghz as well as it being quad-core. A while ago, I visited an adult website and my computer was flooded with links that propped up saying my computer is infected. The only reason why I went on the adult site was because I didn't think I would be in trouble because the guy I got the iMac from told me that it doesn't get viruses. Anyway, I will never go again on an adult website. So all these sites came up while I visited the adult website, one of them telling me my computer wasn't functioning properly and lost strength, or something to that degree. So, I phoned the trouble-shooting people of Apple, where I am entitled for 90 days free support, that came along with the iMac.
The woman on the phone talked me through getting rid of this problem. I ended up having Malwarebytes on my computer, and btw, this is the free version of Malwarebytes, and I ended up getting rid of some files that came up when I pressed scan on the Malwarebytes. After that, the lady said the computer was fine. I have gathered that Malwarebytes scans the computer and detects viruses, is that accurate? I have also read that Malwarebytes can detect viruses but it cannot remove them, which is weird because the woman on phone told me that the computer is fine after I scanned and removed the files after. I spent a lot of money on this computer and I want to be sure that my computer is just fine and completely virus free. A few days ago, I made another scan on Malwarebytes, and one file came up, and I removed it again, even though I did not visit any adult sites, but the lady on the phone said that I can still pick up such threats from other parts of the internet, without having visited adult websites. I want to know if my computer is fine and completely virus free. Now when I press the scan button on Malwarebytes, nothing comes up and it says, "Congratulations, you are clean." Does that mean that I am completely virus free? Also, is it true that those files that came up after I scanned, were removed when I pressed the button to remove, I think the button is "confirm."
Another problem or doubt I have is that, well, recently, I was transferring pictures from my android phone, into the computer. I downloaded a program called "android file transfer" and one transfer was 3.17gb of pictures and it took 2.29 minutes to do so and transfer all of the pictures from my android phone on to the computer, through the program, "android file transfer." I always thought this was too slow and, I suspected, and attributed it to what happened with what happened when I visited the adult website I talked about above, and thought my computer got a virus or an infection that slowed down the computer. I was told that the reason why it was slow transferring both from android to computer was because it naturally and normally takes long to transfer from android to computer.
Another concern of mine is that I was transferring pictures from my computer to a usb and it measured as 3.4gb and it transferred the pictures from my computer into a usb in 5:10 minutes. Again I thought this was too slow seeing that my computer is 3.8ghz as well as it being quad core. At the same time, when I transferred the same 3.4gb from the usb (that I had uploaded it, onto it) to the computer and it did it in 26 seconds. I am concerned.
I would really appreciate any help, if you can help me. Thank you very much.

Malwarebytes searches for and removes malware. I use it about once a month.

The "lady on the phone" is correct, you can pickup malware from anywhere on web, even at generally trusted websites. Most, if not all comes from the advertisements that the site agreed to load together with their content. Adult websites are more likely to load ads with malware, but there are reputable adult sites that don't.

The number of CPUs and its speed does not have much to do with the data transfer rate between the system and the USB interface. The transfer rate in this case determined by the lowest speed between the system and the USB device. The USB 2.0 device has a maximum read speed of ~35MBs, but the USB 3.1 Gen 2 has 1,250MBs using 8b/10b encoding.

The nominal maximum throughput is seldom actually achieved. It is impacted by the file size and the host system's nominal maximum data transfer rate. For example, transferring lot of relatively small files from/to any of the USB devices to the host system will take much longer, than a single large file. Then there's the host system with hard disk instead of SSD, that will not be able to keep up with the USB 3.1 data transfer rate.

In my view, your machine is as clean as it can be with Malwarebytes. If you have some doubts, just reset the macOS by reinstalling the OS from scratch. Just don't forget to backup your data before you do....

Install Ghostery and AdBlock Ultimate in your browser prefewrences.

Well thank you Cr00zng, I have heard that Malwarebytes works better when you also have an antivirus software? How can I be certain that I am completely virus free? Seeing that Malwarebytes works by detecting and removing malware, not also viruses. (?) Malwarebytes is not enough, it seems. And in regard to your comments about USB transfer rate, are you saying that there is nothing wrong with the transfer I made and the amount of time it took? From android to the computer and from the computer to the usb, that is. This is what I hope for. When you talk about reseting the macOS and reinstalling it, are you referring to formatting the computer? Well, I wouldn't know how to do that but I will give it some thought on whether or not I should take it the place where I bought it from, where I have guarantee and they can do it for me. Thank you.

There are no known viruses for Mac products. You did not have a virus. You had pop ups and ads and they are not restricted to adult sites. I stream F1 races and am often taken to gambling sites. NEVER respond to warning messages like this. Genuine alerts from Apple will come come as a personally addressed email or a software update signalled in the App Store.

You should also clear all internet history and cache so that all the sites you visited are wiped from your browsing history. https://www.macworld.co.uk/how-to/mac-software/how-clear-safaris-cache-history-on-mac-3496193/

Ghostery stops websites monitoring your online activity and pushing advertising towards you; Adblock blocks advertising linked with any website you are using.

Thank you badshoehabit, I admit I am a bit of a dummy on this as I never had an iMac before, or any apple computer/product apart from an iPod I bought years ago.
Something else I noticed is that after having downloaded Ghostery and Adblock, when I would scan the computer through Malwarebytes, it takes longer to make the scan, than before. I wonder why. What is the reason?

Can't answer that, but there's absolutely no need to run Malwarebytes so often. You had a scare, you recovered, you learnt lots. Relax and enjoy your new machine.

Malwarebytes once every few weeks is ample.

It is a good idea to clear your Browser History completely from time to time, including cookies. Especially if you are having problems with recurring problem sites or popups. For this reason I do not use Safari as my primary browser. Currently I am using Firefox Quantum which is pretty **** good but the bottom line is you can delete it if you have problems, obviously the same goes for any third party browser. Obviously you cannot delete Safari so I only use Safari for trusted sites and bookmarks I need to keep. So if for whatever reason you have a problem with persistent sites or ransomware, you can simply trash Firefox then reinstall and import your important bookmarks again from Safari. This is a method that has worked well for me over many years and may well be worth a try if you occasionally visit "risky" sites.