Well, 75ms is the time taken without filter which means computing the group by for all 137.000 records.
The query that we can expect for the getter function looks more like this one:
SELECT resource, count(*) from ir_attachment WHERE resource IN ('account.statement,10151', 'account.statement,10160', 'account.statement,1017', 'account.statement,1018', 'account.statement,102', 'account.statement,10213', 'account.statement,10253', 'account.statement,10255', 'account.statement,10259', 'account.statement,10276', 'account.statement,1082', 'account.statement,10847', 'account.statement,10855', 'account.statement,10856', 'account.statement,1089', 'account.statement,10897', 'account.statement,11019', 'account.statement,11046', 'account.statement,11062', 'account.statement,11088', 'account.statement,11095', 'account.statement,11112', 'account.statement,11251', 'account.statement,11263', 'account.statement,11305', 'account.statement,11547', 'account.statement,11557', 'account.statement,11702', 'account.statement,11724', 'account.statement,11857', 'account.statement,11952', 'account.statement,12075', 'account.statement,12076', 'account.statement,12078', 'account.statement,12297', 'account.statement,12388', 'account.statement,12391', 'account.statement,12428', 'account.statement,12440', 'account.statement,99999999') GROUP BY resource ;
and this takes between 1ms and 3ms on several execution runs.