Engine Stuggling to Rev one gear is selected

[OMESH100]

Hi members, my name is Omesh and i hail from Pietermaritzburg, a small town situated in KwaZulu Natal, South Africa. I have a 2013 Isuzu KB 250 LE Double Cab. Please can you assist with following problem i am experiencing..... Once my vehicle is running and when in neutral it revs ups normal. As soon as i engage first gear or any other gear and with the clutch still depressed and then rev the vehicle, the engine then struggles to rev up...Its feels like it has a 5 second lag

Thanking you in advance...