This is probably not helping to solve your problem with performance in mind, but WHY do you use BIGINTs as IDs on ALL your tables? I mean you have a table of 4 values and you used a BIGINT as a DataType for it.

BIGINT uses double the amount of storage than normal INT data types.

Change all your BIGINTs to INTs (even that 1,5 Million rows... It's not big enough to use BIGINTs.

I think there should be a performance increase using only INTs as a datatype, because it will make your indexes smaller, it will make your database much smaller and all this means you use less pages to query from.