Regarding the part that shows you filtering on ‘N’:
This is a chunk that will degrade your performance. If the accepted values for this field are ‘N’ and ‘Y’, and there is less than 30% variety (for instance, if 25% are N and 75% are Y) then an index will be of no use.
Therefore, having the entire database on the ram drive will increase performance.
If the balance between N & Y allows closer to 50:50 distribution, the index will improve performance.
If almost everything is Y with very few N’s, it may be best to hash the table or make a “table of N’s” that is maintained in parallel and can be joined to the original when needed.
Long term solutions involve changes to the data structures. In that case, disk overhead is reduced and the ram drive wouldn’t be necessary.