This takes around 1 minute to execute and returns six rows of data - I appreciate system notes is massive but should this really take 1 minute?

SELECT

name, oldvalue, newvalue, field

FROM

SystemNote

WHERE recordid = 15096

AND field = 'CUSTITEMCUSTID_BS_REPORT_SEAS'

Try giving the record type as the filter. Along with the record id. And just get all fields.