Think of it as a physical mailbox.
Everyday you get a few letters, bills, advertisements (junk mail), and so forth.
Then someone starts sending you all sorts of junk mail to where your mailbox gets full, the postal delivery person has to carry more and stuff to the mailbox, you have to spend more time sorting out the good, the important, and the bad.
Your entire mail performance slows down.
So you try to get off of all those foreign mailing lists which might help.... You try to get the post office to throw those letters away but they really cannot do so. Nor can the postal delivery person decide what should go into your mailbox or into the trashcan you place beside your mailbox.
From the perspective of the internet, your computer is the mailbox. Many people are trying to gain access and/or deliver "things" (malware, viruses, etc.) but hopefully most of those attempts are being blocked by your firewall:
https://www.avast.com/en-gb/f-firewall
(Note: not promoting Avast. You can easily google for other links regarding firewalls and how they work.)
But if as the number of access attempts increases two things happen: 1) the internet connection path to your computer gets busy. There is only a limited amount of bandwidth available. Analogy being traffic lanes - too much traffic slows down internet performance. 2). More time is needed to sort out the allowed traffic to your computer from all of the other unwanted traffic.
Net/End result = slow internet.
What your BF did was just look at the list of senders (sources) trying to gain access to your computer. It seems that, for some reason, there was a lot of attempts being made to do so. Likely not personal.
The bad guys just keep hammering away and trying to get in.
Keep your firewall up, use AV and anti-malware software, use secure passwords to protect your network and computers.
Also, any Wifi network has the added problem of interference and other problems that can slow the performance of your own home's network. You may or may not able to improve on that part of the problem.
Perhaps changing your wireless network frequency and channel may help. But that is an internal factor and not an internet factor.