Slow bandwidth issue on particular devices on same uplink

[J1mb08agg1n5]

Experiencing a weird issue that I'm struggling to explain or fathom, wondering if someone would be able to explain or at least point me in the right direction. Small mobile that is connected to the main building via a single CAT5E cable. It has very minimal device usage with 1 desktop 2 laptops per room and one AP to service both areas. One PC and laptop are completely fine but the other PC and laptop that are in the other room, dead slow. They are both identical in spec and cables go into the same switch and traffic traverses the same uplink into the main building which contains servers, firewall and router. Changed over to a brand new switch as well as patch cables and still same. Also checked 802.1Q VLAN tags on switch ports and all is fine. It is quite a simple VLAN segmentation for wired and wireless devices, they obtain IP addresses fine from DHCP. Cisco L3 switch acting as the gateway for any inter-VLAN routing. External FTTC line and router are a few hops away from main server. Internal and external speed tests show one PC and laptop operates at a crawl, whereas the other is completely fine. Other areas are also OK, it's just this one particular area. From a physical standpoint, the uplink is truncated to an old Netgear switch which when I was doing MTR, iPerf and PathPing tests to it's static IP, it was dropping packets so potentially an issue here but still can't explain why one set of devices have an issue and and another set doesn't. Only thing I can think of is if there is bandwidth limiting or QoS being employed somewhere.

 

Any help would be greatly appreciated.

Edited February 3, 2021 by J1mb08agg1n5