What To capture for a slow site?

I apologise in advance but i'm just learning about Wireshark. I'm an engineer who works for company that does the IT for medical centres. I was asked to investigate a strange issue that has been affecting the site for the last 7 months. Every four weeks on a wednesday, without fail, the entire site grinds to a crawl. There has been investigations by the tech team, and BT, but they cannot seem to find where the problem is coming from. They asked me to run a Wireshark capture. I did this on a day that the site was running fine, and on a day it was crawling. Someone else will get to look at this, but it interests me as well. Would there be anything specific you'd be looking for if you were in this situation? Kind regards. Darren