I thought twice before posting, figuring this was a pile-on for OnX but, considered that maybe this might help someone reading through this thread. First, I don't work for OnX nor am I paid anything by them. I'm just a user that has been using the phone app almost since it first came out. I pay for if annually, just like everyone else. After reviewing all of the comments thus far in this thread, I thought I would share some things I've learned about the product over the years that has helped me have a consistent performance when I needed it.
It's been stated above several times about airplane mode on your phone while in the field. I can't stress this enough. If you haven't disabled "auto-update" for apps on your phone and you're in a sketchy coverage area, your phone will try to download any new version of the app. This can/will cause issues if not completed before you lose cell signal again. Just leave your phone in airplane mode until you're back in solid coverage. Or, turn off the "auto-update" feature for this app on your phone to stop this from happening.
OnX released a new version this year that will automatically download updates to the map layers for any saved offline maps. Thus, once you are back in coverage or, hit a little bit of cell signal in the field, if there is a new update to one of the layers used in an offline saved map, it will try to download it. Obviously, if you fall out of coverage in the middle of this download, the offline map is going to have an issue. Again, disable "auto-update" in your phone and stay in airplane mode to prevent this from happening until you're back in solid coverage.
Having all of your saved tracks and hundreds of waypoints saved to the app is great for in field knowledge and use however, these details are stored on your phone's on-board memory. Not the added memory chip added in the extra slot (if your device has one) but, the actual memory storage capacity on the core circuit board for your phone. This is also the default storage location for any other apps you have downloaded to your phone along with any material downloaded for those apps. On some phones, you can move any downloaded app to the additional memory chip however, anytime the app is updated, the system will usually move the app back to the on-board memory. This is an operating system thing you can't change without some in-depth knowledge of the workings of your operating system. Therefore, if you didn't get the max memory available for your on-board memory on your phone, you are probably going to run out of memory unless you continually perform some memory management and watch your app usage and the number of apps on your phone. Basically, having all that data on your phone uses alot of memory. Take the time to manage this usage on your device and your experience with the OnX app itself will likely improve.
Finally, plan ahead. Download your offline maps when you're in solid coverage. The download speeds have been improved tremendously and I find they no longer take more than a minute or two. Whereas they used to require 30 mins to an hour for each offline map. This is based on being in a 4G network/home WIFI with good download speeds. If you are trying to download offline maps in anything less than 4G, it's going to take a while longer. Also, if you forget and try to download maps in a poor signal area, this will cause problems. And, watch your phone to see if it is in 4G coverage or not. I've been surprised by how many cell sites in remote locations in our area are not even 3G, they are even older technology. Trying to download maps with older technology will take longer and may introduce issues.
And to simply restate this important point again, turn-off the auto-update feature in your phone for the app and leave your phone in airplane mode. These two will cover "most" of your user issues.
Just thought I would try to help some folks. YMMV. Don't yell at me too much. Just trying to help some folks have a better experience.