The network is slow, but only when saving documents

Users say "the network is slow" when they save scanned documents to the document service on SRV1 (10.60.0.15, HTTP on TCP 8080, Linux). A 9 MB upload used to take about 3 seconds and now takes close to 40. The application team has already replied that "it must be the network, nothing changed on our side". PC1 (10.10.5.21, Windows) sits on a user floor; SRV1 sits in the server VLAN behind CORE1.