Extremely slow downloads of 1GB to 9GB files

We've been using WinSCP for several months now and have not had a slowness problem with any size files - until yesterday 12/12/2021. We have slow download speeds from two unrelated ftp sites...Site1 downloading a 9GB file (which usually takes approx. 10 - 15 minutes) has been taking over 5.5 hours, Site2 a 1GB file (which usually takes 3 - 4 minutes) has been taking around 50 minutes. We have been using WinSCP 5.17.10 for several months without issue (until yesterday). After noticing this, we've now upgraded to 5.19.5, but this still occurs. We've scripted the process (via PowerShell) for automation and this is when we first noticed it. We then tried the GUI and noted that the download speed was around 400KB/s (it was usually around 7 - 10MB/s). We tried the GUI several times during the day and at some point the speeds increased back to normal for a few hours...in fact at one time Site1 was downloading at normal speeds (10MB/s) and as soon as that was finished we tried Site2 and that was back to the slow speed (350KB/s). Site1 uses SFTP (port 22), Site2 uses FTP with TLS/SSL Implicit encryption (port 990). Other times we tried today (12/13/2021) both sites were slow, until about 1:00pm then both sites were fast (at least until 2:30pm when the last test was done). WinSCP is run from a Cloud server running Windows Server 2016.
My question is where can we start troubleshooting this ... it almost sounds like the connection changes from time to time during the day (i.e. from a high speed connection to lower speed connection - not sure how that works on a cloud server). We have our cloud vender looking into this too as is the vendor of one of the sites we are downloading from (Site1). One of the vendors mentioned that they've heard that WinSCP has been having some issues with sftp lately, but as mentioned earlier we see this with ftp too. Are there any areas that we can look into?
Thanks

So did you test another FTP/SFTP client at the moments when transfer in WinSCP are slow?

Martin, thanks for your reply, yes we also used FileZilla and had the same results...if one client was slow, so was the other. We had our Cloud vendor check routers and firewalls during the slow periods and nothing stood out. However, by Wednesday night (12/15/2021), while testing, we noticed that the speeds were good again – even after several tests. The next morning we verified that the normal scheduled download process ran without issue and ran more manual tests and all were normal again. Discussed further with our cloud vendor and they did not make any changes to configurations, routers, or firewalls. We can only suspect that since it started on a weekend, and many places update hardware/software during those times, that somewhere a change was made over the weekend (12-12-2021) and was corrected/updated sometime on Wednesday (12-15-2021) by someone farther up the line (our vendors cloud provider? or ISP?). But since Wednesday night (12-15-2021) the speeds have been normal – the situation seems to have been resolved.
Thanks again.