I'm going to answer this as an X-Y problem instead of an answer to your question, given there is already a really good answer here that adresses your question.
When an ISP tells you a certain speed is what you should get according to your subscription, this is indeed a speed that you should be allowed to get on average.
But for consumers, internet works as follows: many users share bandwidth along many households, and that is called overbooking. The idea is that most households don't use up the entire bandwidth at the same time so the speeds can be delivered when necessary, but it also means, that it may happen that lots of people do try to up- or download at the same time causing congestion which can reduce the speeds a little bit.
Because of the last sentence in the previous paragraph, ISP's will tell you that the speeds you pay for are an average and that in most cases you should get that speed.
Unfortunately, when unexperienced employees get this explained, they don't understand the technology, and all they hear is: it's an average, so a lower speed can be expected sometimes.
You have experienced a significant drop to 10-20% of what your connection should be capable of, which is far more than what normally overbooking would cause, which suggests that there's an actual problem on your line.
One thing to note is the following though: If either the up or download is fully saturated, the other will suffer as well. So if you have a 1 MBit upload and 1000 MBit download, but you fully utilize your 1 MBit upload, then the download will slow down to a crawl too. The reason for this is because every sequence of up or download will send a packet back to confirm it was received by the other party, and if this packet cannot be sent back but is queued up, the other side will wait sending you the rest of the data, and thus the connection slows down significantly.
This also means that if for some reason your upload is way lower than you should get, more towards 0.1mbit, you can bet your download will fluctuate a lot and be slow too because the confirmation packets are sent very slow too.
Actions for making troubleshooting easier
The ISP will always want to rule out any local network issues, so it is recommended to use a LAN cable directly from the modem to a laptop or PC (if its a modem/router combi) or to the router behind it and perform the speedtest then. Make sure all other devices are unplugged. If this is slow, call your ISP, because this is the ideal state for you to be in, and it's not working either.
Also, another thing the ISP will want to rule out, is services hanging inside the modem, so always turn off the modem, wait 10 seconds and turn it on again, and see if that solves your issues. When you call them, you can tell them that you already performed these actions. It will tell them that you know what you're doing and they will take you more seriously.
You can ask the ISP to measure the line from their end, and tell them you perform a speedtest with only one device connected to the modem using a LAN cable and that this should give optimal results.
If the ISP says they cannot measure the line, ask to be transferred to a person with more experience, or if they refuse to their supervisor.
If the ISP measures the line, but they claim it is working correctly, they will likely have to send a mechanic over.
>>or configure a script to run it and output to a text file with the same. Throughput issues are often an issue on the consumer side, as ISP bandwidth is pooled so when there is an issue ISP side, they'll generally know from complaints within a few days or so. To rule out consumer side, backup your router config, reset it to default, test, reflash it's firmware, test (test with wired and wireless clients)