John Gill Commentary 2 Samuel 15:33

"And David said unto him, If thou passest on with me, then thou wilt be a burden unto me:" — 2 Samuel 15:33 (ASV)

Unto whom David said After he had heard what he had to say, and what tidings he brought:

and if you pass on with me; in his march and flight:

then you shall be a burden to me; being to be maintained by him; and David having but scanty provisions, and so could not receive useless persons.

As Hushai might be, perhaps an old man, that could be of no service to him, and unfit for travelling, and so would rather be an hinderance than an help unto him.