Q: Monitor problems on Yosemite 10.10.5 on MacBook Pro

I am running my MacBook Pro (15-inch, Mid 2010) with Yosemite 10.10.5. Since loading Yosemite, I have problems with the MacBook running slowly. I think the problem is with the driver for my external LG monitor W2240T. I can't type a 6 letter word without waiting in the middle for the computer to catch up the display with my typing. I am not that fast! I also get large black blocks on the display as well as different colored "noise" blocks. I am guessing it might be the monitor driver. Those are supposed to be included in the Yosemite software.

I don't want to buy a new monitor if the problem can be fixed. I throw myself upon your mercy.   ;-)   I am 73 years old, female,  and worked with computers for 50 of those years. Now retired. I managed a department with unix, IBM PCs, and Macs. My computer knowledge is not real up-to-date, though, so I am hoping for some help.