Requête SQL Lente

SELECT
 [netbios], 
 [application], 
 [utilisateur], 
 [etat], 
 MAX(heureinstall) as heureinst, 
 convert(nchar,substring(MAX(dateinstall),4,2)) as Jours,
 (CASE
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '01' THEN 'Janvier'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '02' THEN 'Fevrier'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '03' THEN 'Mars'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '04' THEN 'Avril'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '05' THEN 'Mai'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '06' THEN 'Juin'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '07' THEN 'Juillet'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '08' THEN 'Aout'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '09' THEN 'Septembre'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '10' THEN 'Octobre'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '11' THEN 'Novembre'
  WHEN convert(nchar,substring(MAX(dateinstall),1,2)) = '12' THEN 'Decembre'
 END) as Mois,
 convert(nchar,substring(MAX(dateinstall),1,2)) as Mois2,
 convert(nchar,substring(MAX(dateinstall),7,4)) as Annee
FROM 
 dbo.installsequence
WHERE 
 utilisateur = 'ABL80068'
GROUP BY 
 [application], 
 [netbios], 
 [utilisateur], 
 [etat]
ORDER BY 
 [application]