Vostro 3420 Slow VPN Performance

We've been purchasing Vostro model laptops for the past few years - mostly 3510's, but now some 3520's and 3420's. We're running into issues with folks using the 3420's remotely and accessing our VPN. 

I've been able to run several performance comparison tests between the various Vostro models and other laptops. There is a consistent slowdown/delay when opening network files ONLY on the Vostro 3420s. The 3510s and 3520s perform normally, taking 15-20 seconds to open files while on our VPN using a cellular hotspot, and 3-10 seconds when on a home WiFi network. The Vostro 3420s take an average of 60-75 seconds to open the same files on a cellular hotspot, and 35 seconds on a home WiFi.

A summary of times/tests:

• Vostro 3420s (Realtek network adapter)
  • File opening On VPN
    • Cellular Hotspot: avg 60-75 seconds, upto 2 minutes.
    • Home wifi network: avg 35 seconds.
  • Ping Tests to Google from home wifi:
    • On VPN: min 37 max 590 avg 197
    • Off VPN: min 13 max 162 avg 23
• Vostro 3520s (Intel network adapters)
  • File opening On VPN
    • Cellular Hotspot: avg 15-25 seconds, not exceeding 35 seconds.
    • Home wifi network: avg 3-10 seconds.
  • Ping Tests to Google from home wifi:
    • On VPN: min 34 max 254 avg 87
    • Off VPN: min 12 max 25 avg 15

I've tried comparing both UNC paths vs mapped drives, and different types of VPN (OpenVPN, WireGuard - no significant/consistent difference  between the two). 

I've also tested downloading files to a local drive and then opening - there wasn't a significant difference between models in the download times. Even on the Vostro 3420, you could manually download the file and open it in a total of less than 20 seconds on a home WiFi network. The main delays seem to be in opening files directly from the network server. 

Is this difference due to the network adapter being a Realtek in the 3420's but an Intel in the other models? Is this a known/expected deficiency of Realtek network adapters? Is there some other significant difference between the 3420s and 3520s? (All are i5)

The Vostro 3420s were purchased for folks who primarily work in the field using cell phone hotspots. They were purchased over 3520s for their reduced size and weight. 

There is no issue with the VPN itself. We have 40+ VPN users and only the 10 using the 3420s are experiencing issues.