@Jwood000, Is this seen across just the Firestick or on devices across the entire network?
If it is only the firestick, I would be more inclined to believe that it may be an issue with the firestick running out of memory. Does restarting the device bring the speeds back to what you expect?
You could try setting the firestick as a high priority device in your router's QoS settings to see if it has any effect.
Also bear in mind that during the evening hours, it is not uncommon to see reductions in speed, as there are more people in your neighborhood and area using the internet and streaming video. Depending on how your ISP has your area's network configured, it may be a result of bandwidth limitations.