Fast WRITE speed or fast READ speed? To move and use apps on its SD Card my droid suggests I use a "fast" SD card. https://www.cameramemoryspeed.com/sd-memory-card-faq/fastest-memory-card/ Ranks SD Cards by WRITE Speed and READ Speed. Interestingly, the same chips seldom appear in both lists. 
If I want to use my SD Card to store and run apps, is fast WRITE speed or is fast READ speed the most important quality to look for.
You should consider the R/W together. Both make differences. However, for the apps, they usually do read and pull the files into memory (RAM) and make use. Write will only be done where something changes or the app's job is to modify the files. Else there will be very less write happening during app's operations. The write will be more used when installing new apps to the SD card. 

So, for most apps, the read will make more difference. Generally, an SD card with higher write speed will also have higher read speeds. In the end, the memory will still be slower than the inbuilt EMMC/UFS in most of the phones. So, some performance drop should still happen.