What kind of connectivity is your employer running for their VPN? In many scenarios, when you connect up to the VPN your traffic will be subject to how fast the path is between you and the VPN concentrator, and it will also be subject to how good of a connection your employer has, and their bandwidth policies. Basically, if your employer is using a DSL modem to connect up to the Internet, the DSL connection hosting the VPN is your bottleneck. For the employer to deliver 25Mbps/25Mbps to you over a VPN effectively, they need double of that (50Mbps/50Mbps) to allow the VPN to not be the bottleneck under the most heavy use cases.
Also, if the VPN uses UDP for transport instead of TCP, packet loss can absolutely destroy your connection speeds. Not only is VPN traffic being lost, data being carried in the VPN is also lost. This can cause problems with retransmits and time-outs in some scenarios.
One final note, if your employer has a slow connection and it just so happens to be saturated while you are using the VPN, performance will drop like a rock. VPNs do not work well on saturated connections or poor quality connections.