Can the use of Lexapro (escitalopram) (15mg) affect cognitive functioning and cause mental slowness?

Yes it can. These are definitely possible side effects of Lexapro (escitalopram). A more complete list can be found at the following website: http://www.rxlist.com/lexapro-side-effects-drug-center.htm Now, riddle me this....I'm curious as to why you chose tag words of "below average IQ."