Many possibilities...

Is the server where pixies is hosted also hosting other sites? Sites that might have had an increase in traffic?

Did the upgrade involve a restructuring of the database? If so, some database queries may not be as efficient as before.

Did the upgrade change the web pages such that they access the database more than they once did? Or less efficiently than they once did?

Is the traffic to the server hitting some limit imposed by the ISP that is then limiting the transfer rates to/from the server for the rest of the billing period?

Has the server's configuration been changed in some way?

I'm not that experienced in these matters but they may be things to consider.

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