1) First you need to back up your data to apple cloud or your computer. 2) To fix both wifi and maintain privacy, go to the privacy setting, and turn location services off, and make sure this is still off when you are done all steps. There is no good reason for apps to know where you are anyway, plus you save energy and increase iphone wireless speed. 3) Make sure your wireless router is set to WPA and/or buy a new ac wireless router. Also, restart your wireless router now and after all changes have been made. 4) Update to the latest patch for iphone 6, don't do this for iphone 5 until or if apple fixes all the bugs. 5) Go to general settings and reset network settings. 6) This will reboot your phone, and you will then need to go to wifi settings and again select your home wifi router and again type in the password. This has currently helped fix the problem (slow wifi speeds and disconnections) for now. If you have this problem again, you may need to repeat step 5. Also, you may need to repeat step 5 after applying a new patch from apple which may or may not fix this problem permanently. You may also need to repeat step 5 if your app store is blank when you load it or if it has any issues.
You may or may not need to also change the DNS on your phone to 8.8.8.8 and reboot the device if the wifi problems persist till the operating system is better patched in the future.
The good news is that these wifi issues are now fixed with ios 9 which seem to have gone back to the network protocols of ios 7. IOS 9 is also a smaller update than IOS 8 and can work on older Iphones like the Iphone 5 and 5s so it is worth upgrading to it if you have an Iphone 5 , 5s , 6, or 6s. Unlike Windows 10, apple does allow you to choose to not upgrade to a newer operating system, and I would suggest learning about it before doing it. Remember as newer iphones come out, the operating systems are then designed for the newer hardware and may cause older iphones to run slower. Thankfully, this is not the case for ios 9. I probably will not upgrade to ios 10 for an iphone 5s if everything works fine on the current operating system especially if an Iphone 7 comes out and I learn that the new operating system was designed for that device. A good way to upgrade to ios 9 is through Itunes by 1st backing up your phone instead of using a over the air update. I would also suggest deleting any apps that you don't need or don't want before upgrading to free up more space. Remember you also want to have at least 75% battery life before upgrading. It seems that with ios8 was the worst operating system from apple in a long time and can only be compared to the bloatware of windows 8. With ios 9 we see apple reversing from bloatware making it more friendly for older iphones.
Once last thing I would recommend to prevent data charges when connected to a wifi hot spot like in your home or hotel. I would go to settings. 2) Cellular 3) Turn Cellular data to off. This will prevent you from using the cell phone network to charge you even if your internet connection on your modem goes down. Also, you can then go to settings then phone to select use wifi calling so that when at a hot spot you are using the wifi to make all calls.