Using Mysql and MongoDB together
What is the data like?  Why does it need updating every second?  Etc.  (Your question is very vague; some of my Plans may not apply, but I can't tell without understanding the problem set.)

Plan A:

If you are SELECTing identical queries more often than you are changing the underlying table(s), the MySQL's "Query cache" may be an excellent solution.

query_cache_type = ON
query_cache_size = 100M  -- No more (unless using Aurora)


The rest is automagic.

Plan B:

Redis or Memcached make better caching tools than another database (eg, MongoDB).

Plan C:

Let's look at your queries and see if they can be sped up.  Better indexes, rearrange the schema, better queries, etc.

Plan D:

Devise some scheme that "knows" whether the data has changed, and avoid re-performing the query.