SD Card Slot speed on MacBook Air

I have a 2012 Model MBA with 128GB RAM. Need more RAM but cannot buy new computer just now. Trying out SDXC card, but cannot tell if it is working properly or not.

Apparently, Class 10 devices have to read/write > 10MB/sec.

Apple advises one can check the max speed capability using About this Mac | More Info | System Report | Card Reader, but there is no speed shown there.

I have a 64GB exFAT-format SanDisk SDXC card in an adapter.

Questions:

1. I see cards rated UHS-1, advertised at 50+ write and 30+ read. Can Mac deal with these at that speed?

2. 64GB cards have to be formatted using exFAT. If I partition the card to two 32GB drives and format with a Mac format, would the transfer rate be faster.

Wide open to suggestions!