What is the minimum MicroSD speed necessary to extend the storage of an Android phone?

tl:dr;

Go in for SDXC UHS class 1 or A1 cards if you are looking to run apps from Ext SD

SD card speeds have two aspects that would impact your proposed use as adoptable-storage ( though your question wrongly assumed that adoptable-storage is available on Lollipop, while it is introduced only from Marshmallow )

• Sequential speeds - which are applicable when one chunk of memory is being written or read, say like a large video or a heavy presentation file




• Random IO speeds - which pertains as the name suggests which are random in nature, as is the case with accessing app or app data , as in the case of adoptable-storage

Till recently compatibility for your purpose was being viewed only with sequential speeds. Motorola recommends UHS class 1 :

• Last year SD card association has come up with a new standard that specifically addresses random IO for application access and is called A1 App Performance

The SD Card Association to Classify IOPS Performance of Memory Cards via Logo

....In today's climate, usage patterns of SD cards by different people vary greatly. Some need to record and store UHD content, other need to run applications, which is why the former benefit from great sequential performance, whereas the latter need guaranteed sequential and random read/write performance

AFAIK, SD cards with this rating are not yet marketed

As additional information here is where the A1 cards stack up when you compare sequential and video speeds

Finally, coming to your question UHS 1 speeds (SDXC) are recommended ( since you haven't linked the cards under proposal, I am unable to specifically say which is better) or if you can wait for a while purchase A1 class which is better suited for adoptable-storage, in case you plan to upgrade

Also see this question for compatibility chart Should SD card be formatted before putting into new phone?