Typically the problem with Fire TVs especially earlier generations is that the device get too many hungry apps loaded on to it and they bring it to its knees. I keep a pretty close eye on the amount of storage space being used and get rid of any app that we do not use often. My guess would be that if you check your storage space that you are low. I have found that if you keep about a GB in reserve that things are much more dependable. If you take off a bunch of Apps and there still is little storage space left then you an app that went rogue ate a bunch of it up and you need to do a factory reset and install only the apps that you use.
We cut the cable several years ago. We started out with a Roku, got a first generation Chromecast and then an Apple TV before switching to first generation Fire TVs and Fire TV Sticks. We have upgraded the fire sticks through the generations as they usually go on sale for ridiculously low prices on Black Friday and Prime Days. We have saved so much money vs Cable over the years that I do not even flinch when buying a new device. Our extended family including several households share subscriptions to various services.
There have been some problems on occasion with every type of “digital media players” that we have used. The people here who are strong advocates for one type or another usually do not know much about the options available. When our expensive Apple TV started having problems and overheating I assumed that it was going bad and retired it. A few months later I read that hackers had used security issues to harness the processing power of huge numbers of Apple TVs for data mining for bitcoins. Apple actively suppressed info on this but came out with security patches When I reset the when I later did a “factory reset” back to its original configuration and the patches were applied, it started working fine. By that time however everyone in our household expressed a strong preference for the Fire TV interface over both the Apple and the Roku.
We had a similar issue with our first generation FireTV Box from some of the apps we installed on it. That thing has so many options for sideloading 3rd party apps that it is not even funny, but there are some minor risks involved which when something goes wrong are typically solved with a factory reset and reinstalling just the apps that you actually use.
The last 4K TV sticks we purchased were $25 a piece. I purchased them to get the newer generation voice remotes which have all the normal functions plus volume and the ability to turn the TV on and off so we don't have to use more than one remote.
Sorry for all the typos...
We had a similar issue with Philo on our original Fire Stick that cleared up with a newer one. However, over the past six months, that newer Fire Stick came down with a bad case of not wanting to reliably connect with the router even when I moved it to be right on top of the router while our original Roku happily connected. At that point, I went with a newer Roku to replace the Fire Stick and haven’t had any connection issues.
Maybe it’s a matter of doing what you do and keep updating the Fire Stick. I’ve also found what you found that dipping under a GB of free space on the Fire Stick is pretty much the ticket to poor performance. None of this is prohibitive to do, but I haven’t had the same issues with either Roku.