User has issues with Remote Desktop to one server but not another

A certain user (let's call him Bob) has reported issues using Remote Desktop when working with a new RDS Server I've set up. He reports "lag" that makes working with this server annoying, if not impossible. A coworker has, indeed, confirmed they saw this lag in action looking over Bob's shoulder. This lag has been described as a periodic freeze of the RDP image (~10 second freeze every few minutes), which seems like a connectivity issue.

Now, this new RDS has way more resources than an old one. It's a newer OS too (Windows Server 2019 Standard vs 2012 Standard), and is actually configured as a proper Windows RDS Server with licensing, as opposed to just a regular Windows Server VM with nothing on it.

Now, Bob has this issue regardless of where he is - he can be at the office using the WiFi, at home and connecting with VPN (we actually have two VPNs, one's using SSTP and the other OpenSSL). The only thing I don't know about Bob is that he's using his own PC rather than a company one to connect, and I've no idea how that's set up (don't ask - there's some office politics involved in this bit).

Since he's the only person reporting issues with the new server (I've been playing around with it, doing some heavy loads and all was fine), and the issue seems to be network-independent I'm thinking this has something to do with his PC or his setup.

Unfortunately, I'm drawing a blank on what that could be - I mean - he's connecting to a new server via RDP - what could possibly cause severe RDP connection lag to one machine but not another, even in a LAN environment?

Alternatively, this could be a connectivity issue... assuming it's possible for a given machine to periodically drop packets when connecting to one server, but not another (all on the same network). But, again, I'm drawing a blank here.