Better to keep frequently used apps on the Android device or move them to an external microSD card? Apps on an Android device can be broken up into three categories:


Apps frequently used, often scores of times per day
Apps used 3-6 times per week
Apps infrequently used


For apps in Categories 2 and 3, it is usually a good practice to move them to an external microSD card when possible (if they don't have widgets you want to use).  This will free up limited memory on the actual Android device.

But what about apps in Category 1?  Apps in that category are read dozens, if not hundreds, or even thousands of times per day.  Is it a good practice to move them to a external microSD card (if they don't have widgets you want to use)?

Here are some points to consider:  


Despite what specs claim, which actually allows faster access: the device's internal storage, or a modern microSD card (Class 10, UHS 1)?
Will copying Category 1 apps to a microSD card cause it to wear faster, thus resulting in a shorter life-span?
These days, it seems likely that a user may want their microSD card to survive many generations of Android devices.  In other words, although many people replace Android devices every 3 - 24 months, most people don't replace microSD cards with such frequency.
Due to their capacity, microSD cards often hold tens of thousands of important photographs and videos.  Yes, backing up is essential, but sometimes microSD cards can fail between backup cycles.  IF moving Category 1 apps to the device's microSD card might have a noticeable impact on the card's life span, it is possible that it may not be a good idea.


So what's the truth?  Is it a good practice to move Category 1 apps to an external microSD card (assuming no widgets), or is it best to leave them directly on the Android device?
It depends on how you use your phone, but under normal circumstances (no task killer apps, small to medium sized memory consumer, etc.) category 1 apps are typically held in RAM. So I guess reading from SD card you will only notice when the app is started for the first time. But beware, that SD cards can outperform internal memory performance: General difference in read/write speeds for internal memory vs SD card

Best advise I can give: just try and use what is best for you. If you experience performance drawbacks, just put the app back to internal storage, if not, than you can leave it on SD card.

Regarding the life time of SD cards, I guess, unless they do not perform to much IO to the card but operate on network and RAM it does not matter. Personally I just put every app I can to SD card, to have more internal memory for apps I cannot move to SD.