I have been watching this for several weeks. It is not anecdotal! It is real and repeatable.
I could be running a 3MB transfer and then click on Peers. It was running at 3MB and suddenly drops to about 40% of that speed. When I click on Pieces, the same thing happens. If I click on Bandwidth, it goes back up. Even if I click on another transfer, it lowers this transfer and that one and all of them. If I click on a link showing Fetching meta-info, it all goes back to full speed. This is completely repeatable and I have been watching it for several weeks and dozens to hundreds of trials. Not a one-time thing. This and the repeating out of memory thing is causing me distress over Tixati! I hope you can fix both.
This is a known thing with Tixati and GUIs and as of late Windows 10 plays its role to slow everything down even more! This has been discussed enough, search for other threads.
In short: The windows are part of Tixati's main processing thread and slowing down their rendering also negatively impacts Tixati. What can you do? Not open these tabs/windows or get a faster CPU that has higher single core performance (~2.5 GHz Intel mobile CPU after vulnerability patches won't cut it).
To fix it I think Tixati needs a huge overhaul and I don't think it will happen any time soon. Such an undertaking needs a lot of motivation to finish too ;)