haven't noticed a difference, however, it may be a percieved difference going from MP to TT - in MP, one is on the 'weapons pickup' racing line, the focus is on weapons rather than boosts, and this can slow you down. On the TT, one is looking only for the speedups and so flies a faster racing line. In MP - try pretending its a time trial and ignore the weapons for the most part, only pickup weapons on the TT racing line, you should see an improvement.

For comparisons, note your race completion times in both modes, this should clear things up.